SOLVED.

[richm0nkey]

I've replaced like this:
PlayMusic("21_loop.snt", true, 10, 0, 0, true);


-- Some more questions may come up, thank you for your help

Should be...

PlayMusic("21_loop.ogg", true, 10, 0, 0, true);

but if I am not mistaken you dont have to play it as music just put it anywhere in your map using the map editor and the player will hear it. This way you can actually play a musical song along with the sound