Early stage murmurs can be tricky and many vets aren't all that experienced in detecting them. If you go to your regular vet and he says he doesn't hear it, it doesn't mean it doesn't exist. It just means it's a 50/50 shot. If you're going to follow up, I would do so with a Veterinary Cardiologist who do this sort of thing every day and can give you a more definitive diagnosis.
Holly, Misty & their friend, Jake, the dog.
Onyx, Callie May, Maggie & Kobi forever in my heart.
Yes, I know I need a new signature pic...someday...