My Cavalier King Charles has a heart murmur. It is now grade 6 so about as bad as it gets, but she is 14yrs old. She is on medication and sees the bet every 6 weeks, but she is doing well. she likes her gentle walks and does not cough at all like the medical books say a dog in her state should. I worry every day but she is happy and pain free. She spends most of her time sleeping on my lap (well shared with Archie anyway), but I put that down to old age, not her heart murmur.
Try not to worry, it can be controlled