That's what the manual says, but in my test script the sound was played in the middle with -100...
In another case the balance value worked as the manual stated. That's strange.

[Edit]Now I have found out more: play_sound works perfectly, if I use stereo sounds! My problem occurs only if I use mono sounds. This reduces my problem to one question: Can I use play_entsound with stereo sounds without reducing the 3D sound quality?