- Open Finder
- Select the Applications folder
- Highlight iPhoto, Control-clik on it and select Show Package Contents from the pop-up menu.
- Once the Contents folder opens in another window, navigate to Resources » Music folder.
- Delete everything from that folder, and insert your favourite MP3 into the folder. This will become the new default. Alternatively, instead of deleting everything, you can also rename your favourite MP3 to something like 1.mp3 and drop it in there, as it will always default to the first track in the list.
[robg adds: "Back in the day," changing the music for a slideshow with the iPhoto Library selected would change the default music for all new playlists, as explained in this hint. However, now that iPhoto creates a special album for each slideshow, this is no longer the case. This recent hint seems to offer a solution for the non-music settings, but it didn't work as described on my test machine. The above solution definitely works, although it's a relatively aggressive way to solve the problem.]

