Converting mp4 to mp3 on a Mac?

I am trying to convert some mp4 (m4p) tracks from iTunes to mp3 to play in Powerpoint on the Mac (OSX). All the converters I can find on the web are Windows or Vista based. Can anyone point me in the right direction?
Thanks to all.

Damn, I just looked up how to do this with iTunes this morning!

In iTunes preferences dialog, importing pane:
Set to “Import using mp3”

Back in the main window, select some tracks.
Under the ‘Advanced’ menu you’ll find an item “Convert Selection to MP3”
-Choose it.

iTunes can do this, for unprotected tracks.
If the tracks are protected, you can burn them to CD, then re-import as MP3s.
The import and conversion settings are in Preferences.

D’oh! I didn’t realize I could do this in iTunes. I wonder if I will have to pay the download fee again.
Oh well. Thanks so much for your help!

It duplicates tracks in the new (mp3) format.
Now I’ve got to figure how to select and delete all the old tracks efficiently.

Make a “smart playlist” with rules to select those you want to delete. “Type contains ‘AAC’” and such. Then select all and drag them to a new normal playlist. Doublecheck, select all, and delete them.