My cat had a heart murmur. Was never given any medication for it. It still caught mice and rabbit and ducks and birds and wiped out a nest of water voles (endangered... Oops).
The murmur was the least of his worries. He led a long happy life despite car accidents, the murmur, the bad diet (my fault for sharing my pizza) and everything else that happened.
So don't worry too much. Animals can still lead relatively normal happy and healthy lives even with a heart murmur, obviously keep an eye for things getting worse but don't worry yourself too much.