I tried inputting some MP3 files into certain game overs for my game The Melanomorphosis, but when I play to those certain game overs the sounds do not play despite having been successful put in. I am a mobile user, which might be the issue, but I'm very unsure.

Not sure, but I think sounds usually don't play on mobile.

There's a restriction in most mobile browsers, that sounds/videos don't play unless you're actually clicking on a 'play' button. Both to avoid triggering annoying noisy ads in public, and because of the potential cost of data.
The exact details of the restrictions vary between browsers; a lot of advertisers have found ways to get around them, but Quest hasn't.

Ah. Ok. That sucks.