For some reason Flatpak is not working in some Linux Mint 12.x boxes.
The workaround is not fancy but here it goes.
From a terminal access root:
$ sudo -i
If using PulseAudio it won't work with root user, so we have to kill it and then load it
# killall pulseaudio && pulseaudio -D && sleep 20 && flatpak run xyz.audiostellar.AudioStellar
The sleep command here, is meant to wait until pulseaudio is loaded, increase it if you need to, or just remove it, wait for pulseaudio to load (check for the volume icon to appear) and then go to Settings -> Audio Settings and start
If you are using Jack or Pipewire, the pulseaudio commands are not needed, just make sure they work as root.
You won't have access to your home files though, which can be really cumbersome. For this you can add permissions to it like this:
# flatpak run xyz.audiostellar.AudioStellar --filesystem=/home/<my_user>
Replace <my_user> with your linux username
If you are using pulseaudio you can also add that parameter in order to access your home files.
Upgrading to Mint 22 will fix the problem without this nasty workaround
