Renamed mp4 files cannot be converted to mp3

I have a list of mp4 files that I want to convert to mp3. After giving them names (e.g. gibberish.mp4 renamed to not gibberish.mp4) they cannot be converted. I've tried several apps, including one I know can convert similar unrenamed files.

from Android Forums at AndroidCentral.com - Ask a Question //forums.androidcentral.com/showthread.php?t=795275&goto=newpost
via IFTTT