It seems I've done everything. I just don't know what all the different settings are and what they do but of all the things I've tried, nothing works.

Wish I could bring my spare set of cheap speakers along and figure this out for you! I keep a set just for this purpose.

Just hooked my speakers to my sound card and set that as default and now my voices are working (so far). I'll update in the future as I see what happens.

This had been the weirdest puzzle! Sure hope the continue working for you!

I made it through Goodbye Deponia without losing my voices, though one day the volume (all of it) went down very low a couple of times for just a short while. I'm probably going to try an older game again soon to see what happens with that because they seem to be more of a problem. I'm playing a casual in there now - they don't seem to ever be affected by this issue.