How to convert that iTunes music (or MP3 file) to WAV or AIFF.
It's a BAD BAD BAD idea to try to edit with MP3 or AAC files from iTunes. Video editors do not like to use compressed sound files (like MP3, etc). Often the music will sound broken or full of static in the video. You need to convert the music to either WAV (for Windows) or AIFF (for Mac) though I have successfully edited WAV files on the Mac, and vice versa.
iTunes: In iTunes, this is very simple, assuming that the music is not copy protected. For music that you've ripped from a CD or is otherwise not copy-protected, this is what you do in iTunes:
Go to Edit >> Preferences >> and select "Import Settings" and you'll see something like this:
Click on "Import Settings." A new window will pop up. Under "Import Using:" select AIFF for Mac, or WAV for Windows. Then click OK.
Find the music you want to convert to WAV or AIFF. Select it and right-click. A pop-up menu will show up. Select "Create AIFF Version" or "Creative WAV Version."
Then wait as iTunes converts it over. When the new track is completed, right click on it, select "Show in Finder" (for Macs) or "Show in Explorer" (Windows) and drag the WAV or AIFF file to your video project folder.
The last thing that you MUST DO is go back to Preferences in iTunes and change the "Import Settings" in iTunes to MP3 or AAC or whatever you had it set to before. If you don't, the next CD you import into iTunes could be converted to HUGE AIFF or WAV files!
To convert copy-protected music:
There are all sorts of audio capture apps for Mac and PC, and other methods, but the simplest is really just to burn a CD of the music, then re-import that music into iTunes. The re-imported music will have no copy protection. This music can be converted to WAV or AIFF (or better, it can be imported into iTunes from the CD as WAV or AIFF.) True, there might be a little bit of quality loss in the re-imported music, but not enough so that most people will mind.