One of our charlies was diagnosed with a murmer when we got him , He very nearly went back to the breeder ( we had lost our first charlie to heart failure 2 weeks before)
Luckily we took him to another vet who found nothing wrong with him , apart from a bit of a snorty nose. He is now 10 & his heart is still ok.
I now take anything vets say with a pinch of salt & don't worry until they show obvious symptoms of a problem
Another of our chalies had a high grade murmer from the age of 2 & lived an active life until he died suddenly at 11