Tutorial on How to Make custom loadloops:

1)Find a Song you want it to play-
Try to find a one under 2 minutes, but any song is fine.

There are two ways you can do this. This is the one I use...
2a)Use a Program such as Adaptec CD Creator 5 Platinum to turn the MP3 into a WAV.
3a)Put the WAV inside the music/modes/load (I think) folder.
4a)Back up the Old Load loop, just in case you want to restore it later.
5a)Delete the old Load loop, after backing it up first.
6a)Rename the song you want to make the load loop, 'loadloop' without the quote marks.
7a)Edit the song as you wish, Quiet songs will not play well so I would increase the volume of it a tad with Sound Recorder (All Windows computers should have this)
8a)You can make it read only, since the original is Read only.
9a)Launch the Sims and see if it works. Enjoy your new Loadloop!

You MUST have a .mp3 to .wav converter.  I've heard it doesn't work as an Mp3.