It's very common. A heart murmur is the sound that blood makes when it flows through the heart. Sometimes a murmur can be caused by a malformation of the heart (tight valves, abnormal holes, etc.), which occur in about 1% of children. Most heart murmurs, however, are normal sounds ("innocent" or "functional" murmurs), which occur in about 40% of children. Your pediatrician or a cardiologist can sort it out.
Murmur. Yes abnormalities of the heart valves are the responsible of heart murmurs and can be acquiered during life not always at birth.
Yes. A heart murmur is a sound you hear with a stethoscope. Many times it can be a harmless "innocent murmur". You can develop a heart problem which will cause an abnormal murmur at any time of life. A cardiologist can tell you if it is a problem.