I have tried but not found how I can just clip out one bit of music from an iTunes song - eg in a Keynote presentation?
If a song in iTunes is copy protected (with DRM), you'll need to burn it to CD and then rip it before you can edit it. If you've got an unprotected track, there are many ways to edit it. Here are a few ways:
1. Adjust track start and stop times in iTunes. If you do a Get Info on the track in iTunes, one of the tabs has a Start and Stop time listed (I'm not at home, so can't say which tab). These default to 0:00:00.000 and the end of the song, but you can adjust these to whatever you want. This will simply clip the beginning and or end of the song. You can then select the song and convert it to AIFF, mp3, AAC (or whatever) to get a shortened track. Don't forget to reset the start and stop times on the original, or it will always play as the shorter version. The biggest problem with this method is that it doesn’t allow for fade ins or fade outs, so the edits will be abrupt.
2. Use Audacity. Audacity is a freeware multi-track recording application. If you open the track in Audacity, you can easily select any section of the song and delete it. You can also apply effects (such as Fade In or Fade Out) by selecting the portion of the song you want to fade in or out, then select the effect from the Effects menu. When you’re done, export the track to AIFF, mp3 or whatever.
3. Use Garageband. Open a Garageband file, the drag the track into GB. If you double-click on the imported track (the actual waveform portion), an edit window will open up at the bottom of the window. You can then double-click wherever you want to split the track. After the track is split, you can click on individual sections in the track section above, then delete the sections you don’t need. Make sure you drag the final clip to the beginning (or there will be a lot of silence at the start of the track). To complete the edit, adjust the volume at the beginning and ending to provide fade ins/outs as required. Then save to disc (under Share menu, I believe).