Poor Sweep
We've had a kitten with a heart murmur in the past, the new owner was worried at first and we had long conversations with her and the vet. There are no guarantees but he has grown up to be a really lovely and active cat, he is now around 18 months old with no sign of illness.
Of course it really depends on how bad the murmur is but often when they're born with these things it can clear up over time.
Best of luck and I hope Sweep has as good an outcome as our kitten.
Fiona