...thanks for that infos  :)
I don't want to risk my membership - and it's so easy, it took about 5min, so if anyone want to reduce any sound of any installed aircraft:

- You need an audio-editor, for example "audacity" is very good and for free. (I'm not allowed to post links, so please google it)
- Open the wave-soundfile you want to edit, e.g.  your flighsim-directory\Captain_Sim\767\sound\apu_start.wav
- Select the whole sound and raise volume with a negative number of decibel., e.g. -20
- Save it and replace the original audio-file, don't forget the backup-copy.

regards and a nice weekend,
Joerg