the file "RCS sustain.wav" isn't playing at all anyway.
(I didn't make a new Orbiter install yet)
OK, that is interesting! That explains why the RCS Attack sound keeps playing: currently, if RCS Sustain is NOT playing, the code will replay RCS Attack because it thinks that the RCS sound just started, and in fact I was able to reproduce what you are seeing (the RCS Attack sound looping continuously) by setting RCSSustain = some_bugus_filename in my XRSound.cfg file. That behavior is a bug that I'll fix shortly with the rewrite of the RCS logic, but that still leaves the question of why the RCS Sustain sound is not playing for you (which, in turn, is what is causing the RCS Attack sound to loop for you).
Given that your XRSound.log shows that the system is not even attempting to play the RCS Sustain sound, that points to the RCS Sustain sound either being disabled or that wav file not being able to load successfully. Can you please check your XRSound.log file and look for a line like the one shown below in red?
03.06.2018 18:16:26.260 - [XR1-01][0.000] XRSoundEngine::LoadWav success: [soundID = 10007, playbackType = BothViewMedium (3), filename = 'XRSound\Default\RCS Attack.wav'] [COLOR="red"]03.06.2018 18:16:26.260 - [XR1-01][0.000] XRSoundEngine::LoadWav success: [soundID = 10008, playbackType = BothViewMedium (3), filename = 'XRSound\Default\RCS Sustain.wav'][/COLOR] 03.06.2018 18:16:26.260 - [XR1-01][0.000] XRSoundEngine::LoadWav success: [soundID = 10018, playbackType = BothViewFar (2), filename = 'XRSound\Default\Crash.wav']
The RCS Sustain sound should be loaded right after RCS Attack, and it should be loaded right when the scenario starts. Do you see any error messages there? Also, can you please double-check that your $ORBITER_ROOT\XRSound\Default\RCS Sustain.wav file exists and can play successfully in a media player? The goal here is to figure out why that sound is not loading/playing for you.