Eh, you can always try recording it. You can grab a program called Audacity and have it record the sound that comes out of your speakers. So stop all other sounds that might be playing (AIM, MSN, your music player, etc.), and you can play the song on YouTube. Then you can hit Record in Audacity and stop it when the song is done. Then you can save your recording.
If you don't mind doing it this way, I'll post a tutorial (or you can always try Google =P) for you later. I don't know how to directly rip the sound from .flv (Flash Video).