I wouldn't have noticed if my son's heartbeat sounded weird through the doppler (first baby), and my OB never thought anything sounded odd. Honestly, I didn't even know you could heart a defect through doppler!
My son's defect (tetralogy of Fallot with a vascular ring) was caught on a level II (my six previous u/s's couldn't detect his stomach, so we knew something was going on, but it wasn't detailed enough to see the heart issues).
My 2nd child was born with mild pulmonary stenosis, but it wasn't detected as a murmur until she was 4mos old. It's a benign issue.
My 3rd child was just diagnosed with a benign murmur at age 3. I still can't believe all 3 kids have a murmur of some sort.
The PC said that as she (#3) gets older they won't detect her murmur anymore, that's b/c the artery walls naturally thicken and you just won't hear it, it's not that it goes away.
And we also talked about how all babies are born with a murmur, as you probably know the valve has to close and that doesn't happen until babies are 1,2,3 days old.
Let us know the outcome of the u/s and if you are really concerned, I would get the fetal echo, there is no down side. I did for all 3 pregnancies just based on my son's history.