Studying the sound.cfg and using a tutorial from google on editing sound, my first discover is CS707 sound consists of 2 engines.

line 4 of sound.cfg
number_of_engines=2

Maybe CS team could explain why?

Anyway I also found out that the idle 4 engines runing consist of several wav files, including avionics, and some exterior sounds. I will have to mute each one of them to see later which needs tweaking in pitch or volume, or replaced altogether.