Okay, so I downloaded the unofficial patch, installed it, everything seems fine so far.

I was wondering about the numbering of the mp3's, though.

The readme for the unofficial patch says one should use 2.mp3 -> 16.mp3.
The soundtrack given by GoG contains 15 tracks... But how should one number them to be able to hear the music the way it's supposed to be?

Thanks in advance for your advice, fellow GOGers. :)

The first track becomes 2.mp3 and the last track becomes 16.mp3. The game should play the appropriate tracks at a certain location. You'll know you did it wrong if one area does not play music.