Update!
I figured it out how to find the assets folder with the help of this friendly neighborhood reddit post: https://www.reddit.com/r/SteamDeck/comments/wzf20a/where_is_the_localappdata_fol..
My assets folder was here: /home/[username]/.local/share/Steam/steamapps/compatdata/[id]/pfx/drive_c/users/steamuser/AppData/Local/VotV/Assets/
I was able to drop some music into the radio folder, and I can verify that it works in game on debian!