I know this is a known bug, but I just want to add my own report, hoping that it gives some additional information that may help in fixing it. On the M7, in scenarios where you have to switch ends on the train, there is no audio from the traction motors after switching ends. But if you go to external camera, and go to the back of the train. (Which was the front before switching ends) that end of the train still has audio. I think that this is an issue of the game not registering that the player is now driving from another cab, meaning the game doesn’t want to play audio because it thinks it would be a waste of processing power. (Kinda like how the game doesn’t render the entire map, only the area you’re in)
Once we receive the reworked lirr update, remember to check it again. Supposedly they did fix the sounds, maybe the issue will be gone once that update drops. If not, all the better to report.
I've just played a couple of runs on LIRR on PS5/PS4 and there are definitely still sound issues. I ran a service from Jamaica and there was no traction motor sounds in the cab at all, even on full throttle. The sounds only returned to the cab after I had stopped at the first station, loaded up and started to move again. All I could hear in the cab was the fan sounds.
Again, they supposedly have fixed the sound issues. The lirr enhancements are currently in beta testing, so it might take some weeks until the update gets released. I would check these issues back after the update has been released.