Good news everyone!
Due to recurring interest, I dug into my old CPC files and managed to find the ST128 player source code.
I put it up on github, for those interested:
https://github.com/BetaSoftCologne/soundtrakker (https://github.com/BetaSoftCologne/soundtrakker)
I will also try to find the sources for the actual tracker and upload those as well, so don't forget to star and watch :)
Cheers!
Thanks to release this old source ! ;D