How old is your "baby"? If still a kitten there is a good chance he will grow out of it, but it is a significant murmur so I do advise you take him to a board certified cardiologist vet. If within reach of Los Angeles, I highly recommend Dr Sarah Miller.
Murmurs are very very very easy to miss. They also can vary visit to visit. A noisy vet clinic makes it almost impossible to hear, a purring or meowing kitten makes it difficult too. My vet takes kittens backstage and finds a quiet corner... and finds a running tap sometimes helps to distract them from purring while listening too!
I've had both ends of this scenario. My first SV was an F2 boy that was 18 months old before his murmur was heard and he was already in congestive heart failure. He'd been to the vet five times without it being heard (both the breeder and my vet). We got him properly diagnosed (another story but the reason I mention it being important that it is a board certified cardiologist) and stabilized and he completely outlived their expectations and lasted until over 12 years of age when lymphoma caught up with him.
The other side is a kitten born here. At his first visit to the vet at 9 weeks of age, they heard a grade III murmur. It was too young to really diagnose but ultrasound showed a lot of turbulence (which causes the sound of the murmur) which was either a significant valve defect or a hole between the chambers that potentially could close over with growth. It was hard to believe he was ill, he was huge and just kept growing at a high rate. We monitored him and by 6 month of age the murmur was basically undetectable and ultrasound looked great. I waited until he was a year old and took him down to LA to Dr Miller. Besides the fact that he has a large heart (she says it is because he is a very large cat) and a very slow heartbeat she says it is one of the healthiest hearts she's ever seen. Whatever caused the murmur when young had resolved.
I think you need to inform the breeder, but not in a way that lays blame. It's completely believable that they thought the kitten was healthy and that the vet had found the kitten healthy. So saying that, if the kitten/cat ends up being diagnosed by a cardiologist with a significant defect/disease, then that doesn't mean that the health/genetic guarantee isn't valid here.
Finding out what is wrong sooner is best though. When it comes to heart disease, many conditions are very manageable with medications so your kitten/cat can still have a great quality of life for years with you.
