itch.io

I know this is probably 7 months too late, but if you're using Pipewire as your audio server, then you should be able to install and use qpwgraph to control audio output in this game, and any other program

there should be similar programs for other audio servers, certainly JACK, but i'm not sure what options there are for ALSA or Pulseaudio, so your luck may vary with finding a graphical interface for easily controlling those