To convert many formats to MP3, and to extract audio from many video formats to MP3 files, one may use FFMPEG:
ffmpeg -i sourcefile.xyz outputfile.mp3
A bash script to create an mp3 from every file in a folder, is thus:
#!/bin/bash
for sourcefile in *; do
ffmpeg -i "${sourcefile}" -ab 192k "${sourcefile}".mp3
done
Notice the “-ab 192k”. By default, ffmpeg seems to transcode to MP3 at 64k, regardless of input; this is not very high quality, whereas 192k does well for general use.