Hi, I'm a vet, and I wouldn't worry too much. Many many puppies grow out of heart murmurs, but your vet has an obligation to tell you that they have heard one- which we hate having to do!
It's really up to you. You would be within your rights to take the puppy back to the breeder and ask for a different one/ refund- in practice this rarely happens, as people are usually already attached to their puppy by the time they get this info.
One of the biggest factors is whether or not the puppy is currently insured, either by yourselves, or still covered by the breeder. If it is (or if the breeder offers to pay! You can but ask!) and the murmur hasn't disappeared in a couple of weeks, you can ask for a referral to a cardiac specialist who can do a special ultrasound of the heart, diagnose specifically the cause of the murmur and tell you the potential likeliehood of it affecting your puppy or not.
The vast majority of murmurs are benighn, but if your puppy's isn't there are all sorts of different options, depending on the cause of the murmur- surgery (not usually invasive, often inserting a balloon into the heart via a blood vessel), medication or just "watch-and-see"
Can I just stress again that most of the murmurs I see in puppies are developmental, and they disappear, leaving the dog to go on and live a long and happy life But if your pup is insured and you are worried, no harm in checking it out fully. Good luck! Let me know how you get on