Heart murmurs are found in both young and older dogs. Usually heart murmurs in young dogs are congenital whereas heart murmurs in older dogs are likely to have heart valve diseases. Heart murmurs are common occurrences in dogs.
Facts about heart murmur in dogs the description of a heart murmur is : "It describes an abnormality in the way the heartbeat sounds when we listen to it with a stethoscope. Unlike the sharp, short beat of a normal heart, the murmur is a softer and longer noise. It is caused by abnormal blood flow through the heart and it indicates exactly that."
In Evaluation of Cough in Dogs with Heart Murmurs by Clarke Atkins discusses in detail types of coughing and how to pinpoint when the coughing is related to a heart murmur."Overall, the most useful tool is the thoracic radiograph."

http://www.vetinfo.com/dmurmur.html provides an easy to read about invidual cases of heart murmurs.

A detailed discussion of heart murmurs with grading."Heart murmurs may be classified as (1) innocent, (2) functional, or (3) pathologic (Table 1). It is useful to describe murmurs on the basis of five auscultatory criteria (Table 2). "
In Cornary Care of Geriatric Dog "Mitral valve disease - disease of the valves in the heart causing heart murmurs........ Mitral valve disease occurs when the valves, especially those between the left atrium and left ventricle, the mitral valves, begin to wear out. Blood leaks back into the atrium instead of being pumped out around the body. This causes a back pressure in the lungs which causes fluid to leak into the lungs.

This is the commonest cause of heart disease in smaller dogs, many of which have the problem by 6 years of age. "

My mini schnauzer Morgy had a heart murmur and was diagnosed at age of nine and was put on enacard a blood pressure medication. He also was given flaxseed oil and coq10. He developed bone cancer and battled it for two and half years and carried his leg for those years and died of heart failure close to 14 years old. He was one tough little doggy.....Very recently I have read that folic acid is great for the heart and arteries. My friend has been on a prescription dosage since his massive heart attack.I have read that up to five m is okay for a dog. Please ask your vet to read up on folic acid and possibly prescribe as a preventative with your dog who has a heart murmur..

J Small Anim Pract. 2003 Jul;44(7):319-25. : Dynamic obstruction of the left ventricular outflow tract in four young dogs.
Connolly DJ, Boswood A.
Department of Veterinary Clinical Sciences, The Royal Veterinary College, Hawkshead Lane, North Mymms, Hertfordshire AL9 7TA.

Four young dogs presented for evaluation of left-sided systolic heart murmurs all showed echocardiographic changes consistent with dynamic left ventricular outflow tract (LVOT) obstruction and subjective evidence of concentric left ventricular hypertrophy. In three of the dogs, abnormal mitral valve apparatus and systolic anterior motion of the anterior mitral valve leaflet with associated mitral insufficiency were also detected. All dogs were medicated with a beta1-adrenergic antagonist. Subsequent examinations showed that the dynamic LVOT obstruction and left ventricular concentric hypertrophy had almost completely resolved. Dynamic LVOT obstruction is a rare condition of young dogs of different breeds. The precise aetiology of the condition remains uncertain. Whether resolution of the outflow obstruction in these four cases was a consequence of treatment or due to changes in ventricular architecture brought about by ageing cannot be established

J Am Vet Med Assoc. 2003 Mar 15;222(6):770-4. : Aortic ejection velocity in healthy Boxers with soft cardiac murmurs and Boxers without cardiac murmurs: 201 cases (1997-2001).
Koplitz SL, Meurs KM, Spier AW, Bonagura JD, Fuentes VL, Wright NA.
Department of Veterinary Clinical Sciences, College of Veterinary Medicine, The Ohio State University, Columbus, OH 43201, USA.

OBJECTIVE: To determine aortic ejection velocity in healthy adult Boxers with soft ejection murmurs without overt structural evidence of left ventricular outflow tract obstruction and in healthy Boxers without cardiac murmurs. DESIGN: Retrospective study. ANIMALS: 201 Boxers. PROCEDURE: Dogs were examined independently by 2 individuals for evidence of a cardiac murmur, and a murmur grade was assigned. Maximal instantaneous (peak) aortic ejection velocity was measured by means of continuous-wave Doppler echocardiography from a subcostal location. Forty-eight dogs were reexamined approximately 1 year later. RESULTS: A soft (grade 1, 2, or 3) left-basilar ejection murmur was detected in 113 (56%) dogs. Overall median aortic ejection velocity was 1.91 m/s (range, 1.31 to 4.02 m/s). Dogs with murmurs had significantly higher aortic ejection velocities than did those without murmurs (median, 2.11 and 1.72 m/s, respectively). Auscultation of a murmur was 87% sensitive and 66% specific for the identification of aortic ejection velocity > 2.0 m/s. An ejection murmur and aortic ejection velocity > 2.0 m/s were identified in 73 (36%) dogs. For most dogs, observed changes in murmur grade and aortic ejection velocity during a follow-up examination 1 year later were not clinically important. CONCLUSIONS AND CLINICAL RELEVANCE: Results suggested that ejection murmurs were common among healthy adult Boxers and that Boxers with murmurs were likely to have high (> 2.0 m/s) aortic ejection velocities. The cause of the murmurs in these dogs is unknown

Vet Rec. 2000 Aug 5;147(6):152-6. : Prevalence of murmurs consistent with aortic stenosis among boxer dogs in Norway and Sweden.
Heiene R, Indrebo A, Kvart C, Skaalnes HM, Ulstad AK.
Department of Small Animal Clinical Sciences, Norwegian School of Veterinary Science, Oslo.

The prevalence and severity of cardiac murmurs consistent with subclinical or clinically detectable aortic stenosis among purebred boxer dogs in Norway and Sweden were evaluated. Two hundred and thirty-one boxers, randomly selected or investigated at dog shows, were examined by phonocardiography by two veterinarians and classified on the basis of the characteristics of their murmurs into categories 0 to 4. No murmur was detected in 23 per cent of the dogs, murmurs classified as category 1 were diagnosed in 25 per cent of the dogs, as category 2 in 46 per cent, as category 3 in 7 per cent cent, and two dogs had category 4 murmurs. In 55 per cent of the dogs, primarily those with very soft murmurs, there was some variation in the intensity of the murmur from beat to beat. The prevalence of cardiac murmurs among Norwegian and Swedish boxers was high and similar to the prevalence reported in Great Britain.

J Am Anim Hosp Assoc. 1997 Nov-Dec;33(6):544-50. : A retrospective study of canine dilated cardiomyopathy (189 cases).
Tidholm A, Jonsson L.
Albano Animal Hospital of Stockholm Tidholm, Sweden.

The case records of 189 dogs (including 38 breeds) with congestive heart failure caused by dilated cardiomyopathy were studied retrospectively. Airedale terriers, boxers, Doberman pinschers, English cocker spaniels, Newfoundlands, St. Bernards, and standard poodles were over-represented. German shepherd dogs were under-represented. A male predominance was observed. Systolic murmurs were detected in 25% of the cases. Atrial fibrillation was the most common arrhythmia. Mild hyperglycemia and mild-to-moderate hypercholesterolemia were found in 38% and 33% of cases, respectively. Histopathological changes consisted of attenuated wavy fibers and interstitial fibrosis

J Am Anim Hosp Assoc. 1996 Nov-Dec;32(6):465-70. : Dilated cardiomyopathy in the Newfoundland: a study of 37 cases (1983-1994).
Tidholm A, Jonsson L.
Animal Hospital of Stockholm, Danderyd, Sweden.

The case records of 37 Newfoundlands with congestive heart failure caused by dilated cardiomyopathy were reviewed in an attempt to compare current knowledge of dilated cardiomyopathy with findings in a large and homogeneous population. The clinical diagnosis was based on echocardiographic findings of reduced shortening fraction (FS) in the presence of clinical and radiographic signs or necropsy findings of left-sided or biventricular heart failure. Systolic murmurs were detected in only four cases. Atrial fibrillation was the most common arrhythmia. Histopathological changes consisted of attenuated wavy fibers and interstitial fibrosis. No significant sex predilection was observed.

J Am Vet Med Assoc. 1996 Jun 15;208(12):2009-12. : Relationship between parental cardiac status in Cavalier King Charles spaniels and prevalence and severity of chronic valvular disease in offspring.
Swenson L, Haggstrom J, Kvart C, Juneja RK.
Department of Animal Breeding and Genetics, Faculty of Agriculture, Swedish University of Agricultural Sciences, Uppsala, Sweden.

OBJECTIVE--To study the relationship between parental cardiac status in Cavalier King Charles Spaniels and development of chronic valvular disease (CVD) in offspring. DESIGN--Historical cohort. ANIMALS--54 female and 53 male Cavalier King Charles Spaniel offspring. PROCEDURE--7 sires, selected on the basis of their liability to develop CVD, were screened for clinical signs of CVD and assigned to 1 of 3 groups (late, intermediate, and early onset of CVD). The mates of these sires (30 dams) were selected and classified likewise, and 107 offspring produced in 1988 from matings between these parents were screened for clinical signs of CVD at a mean age of 5.3 +/- 0.3 years. RESULTS--55% of the offspring were free from clinical signs of CVD, whereas 45% had cardiac murmurs of low or moderate intensity. The proportion of offspring with heart murmurs and the intensity of murmurs were significantly greater with increased parental classification. More males than females had developed murmurs, and murmurs of moderate intensity also were more prevalent in males. Results of multiple-regression analysis indicated that mean parental classification and sex had significant effects on proportion of offspring with murmurs and their intensity. Additionally, age affected disease prevalence and severity, despite the narrow range in age of offspring examined. CLINICAL IMPLICATIONS--Parental CVD status is an important factor influencing the probability of heart murmurs and their intensity in offspring. The results of this study indicate that CVD development is a polygenic threshold trait and that sex of the offspring influences threshold levels.

Zentralbl Veterinarmed A. 1996 Apr;43(2):103-10. : Echocardiographic study of mitral valve prolapse in dachshunds.
Pedersen HD, Kristensen Bo, Norby B, Lorentzen KA.
Small Animal Hospital, Department of Clinical Studies, Royal Veterinary and Agricultural University, Frederiksberg, Denmark.

In this prospective echocardiographic study, we investigated the occurrence of mitral valve prolapse (MVP) in 60 dachshunds: 30 with mitral regurgitation (MR), 15 age-matched and 15 3-year-old controls without heart murmurs. To assess the MVP, video recorded sequences from the right parasternal long axis 4-chamber view were blindly evaluated by three observers. Of the 30 dogs with MR, 12 (40%) had severe MVP, 10 (33%) had mild MVP, and eight (27%) had a normal mitral valve. The clinical status of the dogs with MR correlated significantly with the severity of MVP, and these dogs had significantly worse MVP than age-matched controls, among which seven (47%) had mild MVP and eight (53%) had a normal mitral valve ( P < 0.01). In the group of 15 young dachshunds without heart murmurs, seven (47%) had mild MVP and eight (53%) had a normal mitral valve. The degree of MVP correlated significantly with the occurrence of arrhythmias, particularly severe sinus arrhythmia. We conclude that dachshunds with MR have a higher prevalence of MVP than controls, and that the severity of MVP is correlated with clinical status. The dogs with MVP and marked sinus arrhythmia might have autonomic dysfunction, analogous to findings in humans. Whether young dachshunds with MVP are predisposed to MR later in life must await the results of a longitudinal study.