Wow, how unlucky that both ducklings had heart murmurs, that's very unusual, though I know ducks sometimes can develop heart problems. I knew a wild duck that showed similar symptoms when he was young, too. He managed to get better on his own and live into adulthood, though, but was never as...
If he's walking around and moving normally, he may be OK and just sore from his fall. And, he might be going through a growth spurt and need extra vitamins.