Yes, you’re not going crazy, there are definitely audio dropouts on Redream! I wonder if the reason it runs well on Retropie has anything to do with Retropie remaining a 32 bit OS? Maybe someone cleverer than me could say.
Another good option is the standalone Flycast core, which is actually my main go-to for DC on Batocera at the moment. It’s not perfect and you still have to set the window to 640×480, but it’s better than the libretro version for some reason. It still struggles with the likes of Daytona 2001 and Tennis 2k2 but is’ overall a good choice.
I suspect as DC emulation improves, the sad reality will be the Pi 4 is not really powerful enough for those more difficult games. Still, if we can get rid of those audio dropouts on Redream then that would be more than good enough imho.