Yupp, you're right. Leaving scummvm via softkey screws the sound again. Only way to make it work again is leaving scummvm and doing the trick on the next startup.

Something seems to happen, while softkeying out of scummvm.

You could try changing some of the audio settings. It seems to works semi regularly for me when I increase the output rate and subtitle speed. That hint really did it for me. I changed music driver from default to Adlib and output rate to 48 kHz. In addition I checked Mixed Adlib/Midi mode in MIDI ...

This is exactly what I get on my 6220c (the borked video). But restarting ScummVM doesn't help to fix the sound for me.