For AVI files, <a href="http://prdownloads.sourceforge.net/virtualdub/VirtualDub-1.6.11.zip?download">VirtualDub</a> gives you the option of saving the WAV stream, and from there you can use any number freeware/shareware audio converting applications to turn that into an MP3 file

<i>File -> Open video file...</i> then select your AVI file.

<i>File -> Save WAV...</i> to save the WAV stream.