Or just use iTunes' built in one! Go into preferences, and under the 'advanced' tab area, set it to MP3, with the quality you want. Then select all the songs in your library which are MP4, and right click > convert to MP3. You can search for all the MP4 songs by right clicking on the column headers in the library view and ticking 'Kind' and sorting by that column. Depending on how many songs you've got, you may want to go and do something else for a bit. It can take a very very long time.