@alphabetapie As of the current retroarch version, I don't think you can set the audio output gain per game (correct me if I'm wrong!). It's a system setting. I think the first step would be to find out if you can load different system settings depending on what game you launch.
If that doesn't work, you could do it with this step (although I don't know how retropie manages retroarch.cfg since they have their own scripts that modify settings):
You could implement a script using sed to change the
audio_volume in ~/.config/retroarch/retroarch.cfg before launching the game from retroarch. Maybe there would be a way to do this in the menu before retroarch is launched with retropie. Changing the audio_volume in retroarch.cfg would be trivial with sed, it would just be a matter of figuring out how to change it depending on what game you would want.
Thanks for all the helpful info and trying to assist. I guess the best I can do at the moment is to just adjust the audio volume in the arcade game's dipswitches for now and see if it persists. It is what the arcade owners would have had to do back in the day using that method.
(It didn't solve the weird sound error notification, but hey if it works it works!)
I think that sound/alsa error comes from EmulationStation, not fro the emulator. Use the EmulationStation menu and play with the Sound settings - Audio Card/Device are the ones that produce this error. Most likely you can change the Audio Card from Default to something else and the error will go away.
@DieKatz I just wanted to say thanks for noticing this problem. While playing Sonic 3 on my gpi I noticed that some of the sound effects weren't playing at all. I could never figure out what the problem was. It was the same stereo/mono issue as you were having. So thanks again!