In QuickTime 7 there is a SAVE AS feature, that includes AUDIO as MP3 or so.
In MPEG Streamclip there is an option under the FILE menu to SAVE AS AUDIO, where you can set it to me an .mp3, but MisterGGJstudios' recommendation is probably the easiest, though I have never used that.
If you download YouTube videos, you can try ClipGrab, it gives the option to save as MP3.