MP4 audio files?
I only have MP4 Video files, but
foobar2000 converted all of them to MP3 audio with File/Open/Select All/Open, then right click/Convert/Quick Convert/MP3 and done. About 20 seconds if they're all in the same folder. Then quite a while for the actual conversion, but you only need to kick it off and let it run.
I suspect it will do the same for MP4 audio.
It's my go-to tool for pretty much any file type conversion. I recently converted a hundred+ SACD and DVDAudio albums to FLAC 5.1 for my new (to me) car.