I have a monitor with awful speakers (thanks Acer), and SteamOS will dutifully switch to them instead of using its own much better speakers. I made a non-steam shortcut with this abomination to kick it back to the internal speakers.

pactl set-default-sink 'alsa_output.pci-0000_04_00.5-platform-acp5x_mach.0.HiFi__hw_acp5x_1__sink'