how to convert youtube video to mp3

The fact that you're using Tiger bars the most obvious solution - Audio Hijack Pro (you would play the video and record the audio output to an mp3 file in real time).

This very old thread offered a solution but I don't know whether the program mentioned is still available.

https://discussions.apple.com/thread/569722

http://www.fraction.jp/log/archives/2008/01/18/FLVMP3Extractor