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There are some inherited and congenital defects that lead to abnormal heart sounds. 

In young cats, a veterinarian will sometimes hear benign (also termed innocent, functional or physiologic) heart murmurs. They are produced because the young strong heart is beating in close proximity to the chest wall, and we can therefore hear the strong, normal turbulence produced from blood flowing in the heart and large adjacent vessels. These soft murmurs may be identified at the first check up, and may vary in intensity depending on whether the kitten is standing, sitting or lying down. These kittens will be thriving and growing well. Follow up stethoscope assessments during physical exams will track the murmur as the kitten matures. ECG, X-ray and echocardiographs will also be normal if it is a physiologic murmur. Benign heart murmurs usually resolve themselves by about three to six months of age. 

Significant murmurs will produce changes in the ECG and other tests, and birth defects of the heart may produce stunted and/or unthrifty kittens. Though significant murmurs may require surgical or medical therapy. Prognosis depends on type of inherited or congenital defect, and its severity.

Mitral valve dysplasia is one of the more common cat conditions. This valve defect results in a leaky valve and the murmur results from the turbulence of blood reflux.

Pulmonic stenosis is a narrowing of the outflow area of the pulmonary artery at the exit from the right ventricle of the heart. The murmur reflects the abnormal turbulence that results where the outgoing blood meets resistance. Signs correlate to the severity of the narrowing of the artery. Aortic stenosis is a similar narrowing but occurs in the aorta outflow area. The murmur develops for the same reason.

Ventricular septal defect (VSD) is a hole in the interior wall of the pumping chambers. If the hole is very large, murmurs may be absent because lower turbulence occurs. 

Patent ductus arteriosus (PDA) is a lack of closure of a duct in the fetal heart that will produce shunting of blood in the heart when it remains open after birth. If shunting occurs from the right to the left, usually murmurs are usually absent, but in the usual left-to-right side PDA shunt, murmurs can be heard. Cats may eventually succumb to heart failure, though they often look fine until late in the progression of that failure.

Tetralogy of Fallot is a complex defect affecting more than one segment of the heart and emerging arteries. This murmur is usually a result of the pulmonic stenosis portion of the defect.

If a murmur is detected during examination of your cat, discuss the issue with your veterinarian. Cats are very good at hiding problems, so follow up by a veterinary professional is very important. Your veterinarian may refer you to a cardiology specialist if a defect is confirmed.