If you bought the game on GOG, you noticied something : some musics are missing. Rayman Forever is a compilation, the original game +his add ons. When the game was released, there was not enough available space on the CD to put all the files, that's why some music files were "cut", and that's why there are "missing musics" in GOG's version of the game.
Fortunately, it's very simple to solve that problem. Just follow those steps :
1) Download that archive :
It contains the complete soundtrack of the game in an exploitable format for Dosbox. As you can see, there is the same number of files than in GOG's version but the difference is that in the original soundtrack some tracks got 2 or 3 differents musics in it : 1 audio file can contain 4 minutes of music when GOG's version of the same file contain only 1 minute 30.
2) Extract the music somewhere.
3) Copy or cut the files you just extracted.
3)Go to "Rayman Forever/Music" folder and past the files here (you can replace the previous ones)
4) Now you just have to launch the game and to appreciate the game with its original soundtrack.
I hope it will be usefull for you.
Post here if you encounter any problem during the download/installation of those musics.
Note : a lot of people are complaining about the fact that in this version the music completely loop whatever level you are playing. This is NOT a bug ; there were different versions of the game depending on the country the game was sold. Some countries had the game version where each level has it own track, and other countries had the game version where all the tracks where playing no matter on which level you actually are. Here's what wikipedia says about it :
"Most PC versions of Rayman had the music and backing SFX tracks for each world combined into one CD audio track, which was repeated throughout most levels within those worlds; including the data track, this made for a total of 20 or 25 (multi-language versions) tracks. In contrast, the PlayStation and Saturn versions contained each track as a separate CD audio track, and changed the track played depending on the sub-level and its intended atmosphere; this totalled 51 tracks."