Yeah, I think I know what happened. The reason for the audio bug and the reason it was never noticed during testing is it only affects some PCs that don't have the appropriate language settings installed. You don't have the 'é' in Pokemon, which causes the BGM files to fail.
It's a really simple fix, and all you have to do is make sure the audio file is the EXACT name that I've been provided below. You don't actually need to download the ogm files. You have to make sure the name has that é in it. It's usually these two files:
Pokémon Sword & Shield - Gym Leader Battle Music (Full)
Oldale TownLavaridge Town (Pokémon Omega Ruby & Alpha Sapphire OST)
If you can't change or delete the audio file, make sure you don't have the game/file open somewhere else? Or just try re-downloading the entire game again and then editing that version? You can also try running the game in the itchio app, too. https://itch.io/app
Hope this helps!