This is the easy way of changing the Sims 2 music for your loading screen when you start-up your game. SimPE is not required.
1- Open the following files: C:\Program Files\EA GAMES\(The name of the Expansion or Stuff you want the music from)\TSData\Res\Sound
2- Copy the the package named Splash.package
3- Rename the package by giving it a number, for example: Splash0.package
4- Place this in the following location: C:\Program Files\EA GAMES\(Latest EP or SP)\TSData\Res\Sound, but before you do that, make sure that you remove (or make a backup of the Splash.package you want to replace), then paste the one you renamed and copied from another EP or SP into the your latest EP or SP folder in here: C:\Program Files\EA GAMES\(Latest EP or SP)\TSData\Res\Sound.
Note: When to change the music again, simply do the same procedure, however, you need to rename the Splash.package with something else other than the same text or numbers you used previously.