oh thank you very much !
As I told before, I tried to convert Delphi version to Lazarus. Of course I had to correct some mistakes due to conversion imperfections (like "wrong number of parameters"... etc), but I saw "thread" stuff and my programming teacher told me to abandon, because it was far too complicated. But I'll try with this link again, if it works directly with Lazarus.
In fact I don't need all features of ACS (I might only need AudioMixer). What I need is a "mod" that changes a .wav file into an array of integers (or reals whatever) so I can easily mix two sound (by adding two arrays) and concatenate them. I'll create a new topic...