Have you uploaded a movie to YouTube and then lost your copy of the song?
If not then this is probably illegal (I wouldn't know, I haven't studied law).
1. Find the song you lost.
2. Copy the URL from the address bar.
3. Open TubeTV.
4. Paste the link into the Address bar.
5. Push the "save" icon.
6. Once the file has downloaded, open the file in Quicktime Pro (with Perian installed of course).
7. Export the video as an aif.
8. Open the exported file with Switch.
9. Select the mp3 (or whatever you want) option, the location to save the file and click convert.
10. That's it!
(For the Applications mentioned, refer to previous posts.)