The software that comes to my mind to extract the audio track of an MPEG-4 file is Quicktime Player 7. Open the file with QP7 and choose File>Export and then choose Audio to Aiff, Wave, or AU from the popup menu next to Export. Quicktime player (the default in Snow Leopard) doesn't have this feature, but if you installed the Quicktime Player 7 (that's an option when installing Snow Leopard and will then be in your utilities folder), you should be able to do this. I don't know if this is a feature solely in Quicktime Player 7 Pro or not; I think not, but not sure.
Another app worth looking at is
Amadeus Pro. If you open the MPEG-4 file win Amadeus, it will open the audio track and you can then save it out in any audio format you like. Amadeus Pro has many other audio editing features as well.
A third (open source) app is
Audacity. It has many of the same features as Amadeus Pro, but I find it less stable. Still, it will open an MPEG-4 file and save out the audio portion as an mp3.