I have the same problem, "Failed to play" , "Storage unmounted".
I'm running Andriod 12, security patch level Dec 1, 2021.
My Poweramp (version build-920-bundle-play [920004-a387e829]) does not have a "Settings > Misc > File Access Legacy Mode" entry!
What now?
In the Android App permission settings, I seem to be unable to give storage permissions (just an idea), this is not listed there.
Help!
EDIT: I tried Android Settings / Apps / Poweramp / Storage / Clear data (which should clear the cache too). Now it works. Sadly, all my Poweramp settings and my playlists are gone.