My recorded BGM and BGS won't work.

[Notchella]

I'm just messing around right now to learn the engine and recorded some Zelda music to just use as filler for now. I had saved the music both as a .mp4 and a .ogg in the correct folders but the mp4 won't show up (not sure if that's meant to happen) and my .ogg sound file won't play. It'll play in my normal sound/ media players but not in RPG maker itself.

What am I doing wrong here?

 

[yamakoku]

From what I know, RPG Maker MZ uses only OGG. Previously, MV used both OGG and M4A depending on the platform, so that's probably where you got MP4 from, but it doesn't seem to support M4A either now. MP4 is also primarily a video format while MP3 is audio based.
As for OGG not working, it could potentially be a glitch that occurs when you playtest converted audio files too much. Try closing your program and opening it again.
TL;DR: MZ only supports OGG, try restarting and trying again