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).the file "RCS sustain.wav" isn't playing at all anyway.
(I didn't make a new Orbiter install yet)
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']