I've used the Windows version of Nestopia and scroll/sound sync is PERFECT in full screen, with vsync on.
In the MAC OSX port however, it's not the case. I've tried every combination (no frameskip / auto-frameskip, vsync, etc..)
and I get perfect sound OR prefect graphis, but NEVER BOTH. Perfect scroll causes minimal but perceptible sound skips/repeats.
Other MAC versions by Bannister also have the same issues: Genesis Plus, for example.
I am on a G4 computer, by the way. It's NOT a speed issue, but a sync issue.