The three main music samples live in your /GP4/Sounds folder. They are called tk1.st1, tk2.sk1 and tk3.st1.
To edit them, you need
GP4 Sound.exe and some new music in .wav format.
Convert one of the existing .st1 files back to a .wav first to see exactly which format you need your new .wavs to be in (because not all possible .wav options are supported).