Cool, you fixed my issues!
The audio output still doesn’t default to the deck speakers, but after following your hint it is now at least easy to select it while in gaming mode.
I installed the decky loader, and as non-steam game Audiotube. It seems to work, doesn’t matter very much to me if keyboard shortcuts like next track don’t function, I usually listen to full albums or dj mixes of 1 or 2 hours.
Played it pirated years ago. Not really looking to play again tbh because it’s so sad, but I’ll buy it discounted for supporting, I back then liked the very original angle of playing with trapped victims instead of army.