Project [Name] > audio > bgm (or any of the others)
- This is where it needs to be saved.
You need BOTH an OGG and M4A formated music file. I've tried with just one and it doesn't work.
Audacity should come with OGG, but you'll have to aquire the M4A format from a codec update. To download this codec, open Audacity and use Ctrl+P to open Preferences. Go to the tab labeled "Libraries" and it will show a section titled, "FFmpeg Import/Export Library." You will need to download it, as Audacity cannot legally come with some codecs. The Audacity webpage will open and tell you how to install the codec properly. You can check to see if it has been installed properly by going back into the Preferences > Libraries and using the [Locate] button. If it is automatically detected, it has been installed correctly. The "working" version I currently have is F(55.33.100),C(55.52.102),U(52.66.100), so if you have the same, then exporting as M4A should work correctly.
While I haven't entirely experimented on this part, attempt to convert from ANY* to OGG then to M4A. Save both a copy of the OGG and M4A into the audio directory it belongs to.
*Any cannot include M4A. From personal experience, it must be OGG to M4A.
Once you have a copy of both an M4A and OGG, you don't need to close RPG Maker MV for them to show up. Just save the project and the audio database will update.
Finally; I too have experienced audio files not loading properly but I have found some reasonable explanation for this. Any music or sound that lasts for a long time will not work... again, from my own personal experience. I had an hour long loop of background music I wanted to use, but it would not work properly, if not at all. Sometimes long audio files do work; however, they will have skips and delays, especially when booted. You might not get sound for a couple minutes and it might end early as well.
Hopefully any of this information is helpful.