The linux audio situation for this game is a joke at the moment, the steam deck uses pipewire as its audio server and lacks the pulseaudio binary, and the way the game checks which audio server to use is very naive, it's done by just running pulseaudio --check and of course that doesn't return 0 because the binary doesn't exist so it defaults back to using ALSA instead of pulseaudio, even though using pulseaudio would work fine since pipewire-pulse is installed,
I reported this here many months ago but no fix