Im not pointing fingers at anything else on retropie or anything you've done, I just think their version of AM or their method of installing isn't right for RPi4. I'll get you the install script for manual AM compile plus its dependencies and ill bet the sound issue will go away. When I first tried to get AM running on a Pi4, I had a lot of issues, but once I got the right compile setup its been smooth sailing.
I'm sitting in the car while my kids finish their sledding in 5* cold. Once they tire out or freeze ill post up those compile instructions.