Regarding MP3gain...
Don’t be afraid of it. Any changes it makes can be undone.
I decided to make all my tracks have a gain of 80.0db. It’s been so long since I made that decision that I can’t remember why I decided that, but it’s worked.
Get all your tracks converted from mp4 to mp3 and stick them in a folder. Use “Open folder” in MP3gain. Do an analysis. Then set the gain to your preferred setting and click track gain to set the gain for all the tracks.
Easy and quick to use.
How do you convert acc to mp3?