What a great vet! my neighbor's vet missed her dog's murmur and tragically ended up dying at 3 yrs old. There are different reasons for murmurs, a common reason in human babies is called, patent ductus arteriousus (PDA). If you read the link, it is very treatable. Sounds like you dog is in good care.
What Is Patent Ductus Arteriosus? - NHLBI, NIH