Sounds like there’s audio channel issue somewhere (pun intended) where the game is processing the audio to a specific channel that your speakers and TV are not mixing. So it’s only making sounds for the channels they’re using and since the ATC is using a different channel, it’s only capturing the sounds that bleeds into that channel, sounding faint.
Try to check your Windows audio settings, and change the channel setup back to Stereo. And make sure the spatial sound in the sim is OFF.