Thanks for all the suggestions...
@ Mr. Peanut:
I have one of your missions unPBO'ed here on my HD. I looked at your nitro.sqs, and description.ext to learn, how this can be realized. I will use only a totally simplified once-available-boost for my cessna.
Anyway, i fixed that - say "sound" - now by myself. Guess what...
... the say command by using .ogg file format works only for mono sounds!
Here is what i found out about .ogg files in OFP:
playMusic "music" --> works for stereo and mono
playSound "sound" --> stereo or mono is possible
say "sound" --> works only in mono
Maybe this can help other mapmakers, who have trouble with the sound files and the - say - command.