Some time ago I adjusted the sound settings in the TSW main files and have been experiencing decent sound levels in the cabs of the services that I am driving. Today was my first in a 166 and it reacts strangely, if you are facing the front and driving all is quiet, look to the left or right and you get the sounds back. When passing another service you get sounds even when facing the front.
Does your sound setup in windows corresponds your actual sound system? Class 166 is quite unique in its sound setup as almost all of the sound in cab comes from behind. Other locos are more balanced and provide some ambient from the front. So if your speaker configuration is set, for instance, to 5.1 or 7.1, but actual sound system connected as 2.0 (stereo or headphones), TSW will send sound to non-existent rear speakers. I'm offering this solution because I use surround sound system myself (and it is awesome in TSW) and quite often forget to enable rear speakers, getting exactly your result.
Nope, set up as 2.0 stereo so all sounds to front speakers. I will say that in the cab of the 66 I experience the same issue with engine noise but there are great ambient sounds for the cab no matter the 'head' position. It rattles and bang like you would expect a well used freight motor to sound and there are great track noises.
I have noticed this as well. Nothing facing front turn 1/4 turn and you get sound and loudest facing towards the rear.
I wonder what causes this in your cases. I have no problem getting sound facing any direction. It comes from back speakers in 7.1 mode and from front ones if I switch windows sound config to 2.0, still perfectly audible. Not using an on-board audio chip though.
I have exactly the same issue, edting the sound settings solved it a little bit. I'm using a Logitech G933 headset and it has nothing to do with 2.1/7.1 I guess because the issue is only with the 166, not with other trains. I reported this already months ago in a ticket by the way....
