Last year, a friend sent me some audio of a conference he attended. I was glad to have it, but it was a single, 40-minute track. That would have been a perfect candidate for this tip.
Lifehacker even explains how to convert multiple files at once. Check it out.
If you want file conversion that's more powerful than this simple trick, consider Switch by NCH Software. Switch converts a great number of file formats into any of several options. For example, convert aif/aiff, gsm, vox aac, mp2, or m4a (plus a lot more) into Mp3 or wav. You can even import video files and extract audio (avi, mov, mpeg). There's both a free and paid version of Switch.