It depends on the severity of the heart murmur. It is not uncommon for a puppy to have a minor heart murmur and grow out of it. They will often show no symptoms. If it is more severe, there are heart medications for dogs. A dog with a heart condition has most of the same signs that people have: low energy, cough, out of breath fast. Ask your vet how severe your puppy's murmur is.

my boy sheltie has DOUBLE heart murmurs- one each on both ventricles. The breeder and vet said he would only be able to survive to 3-6 months old. He will be 11 years old 4/15/11!... He's a little slower and chubbier than my "healthy" girl sheltie, but he has his own blankie, pillows, and many, many toys which he enjoys. He has always had a very full, happy, spoiled life. And, I think because of all the the extra care and attention he's been allotted- Tristian is the sweetest, smartest, loyal little boy in the world! (He doesn't act or think like a dog). He is a very evolved soul.
I must add he has always had excellent and copious vet care- including a cardiologist.
Hope this helps.
Heather, Tristian, Brynna
httillette@yahoo.com