Yes is in possible for a puppy to have a small heart murmur, and then go away as the pup gets a bit older. It is possible for puppies in all breeds. I would ask the breeder if they could give the vets info, so I could talk to the vet about it. If it is gone, you are probably fine.
I would ask the owner to have an echocardiogram done before i would purchase the puppy. while it may be an innocent heart murmur ( one that will go away) Cavaliers are known for heart problems ... severe heart problems ... and i would want to make sure it is not what they are genetically predisposed to before i would take the puppy into my home.
Cavaliers and known for heart problems and I wouldn't touch one that already has a murmur. The parents should be checked by a cardiologist yearly. Make sure they are at least four and have clear hearts before even thinking of buying one.
I do not know the cavalier breed, but I have been educated in an intensive way regarding heart murmurs in puppies. We had a pup born in October with a 2 mm hole between his ventricles. It was misdiagnosed twice and a trip to Colorado State University Veterinary Hospital diagnosed it accurately. The heart specialist there are innocent murmurs that can repair themselves, but said most murmurs never go away. As the heart matures, the walls thicken and can "absorb" a ventricular or atrial murmur that is in the wall tissue. It essentially is still there, just covered with heart muscle. Most murmurs can be detected with a stethoscope, so you can have the pup vet checked also. I agree, they should have an echocardiogram and provide it to you. The cost we found was $320-$380 from veterinary surgeons, but CSU charged us $220. You should know if it is/was an opening, a valve murmur, the size, etc. Our cardiologist gave me a full report, with 9 echocardiogram pictures. I sold him for a reduced price with his entire medical file going with him and a contract stating that if anything should happen to him b/c of his heart, I would put their purchase price toward the price of a replacement pup.