List of file sound extensions

[madscijr]

Wow, that adds up! For m4a and mp4 files, I'd just convert them. I've been using VideoProc Converter AI for audio/video and it works pretty well, but it isn't free.  Any recommendations for a good free media converter?

https://www.videolan.org/vlc/  <-- There's nothing better, and it'll run all your audio/video needs, without  having to download any codecs or anything else.
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(Note that the video there doesn't show how to set bit rates or channels and such, but those are customizable as well in VLC.  Just click the  little wrench icon for settings near the mp3 name/type when you choose mp3 as your converted type.)

I can vouch for VLC as a player - been using it for years, and it can handle anything! 
I haven't used it to convert file formats, but did try swapping in a different audio file for a video soundtrack and it worked. Good call.