I'm having the same issue as above. The sound drops in the same area although a little earlier on than others have mentioned.
Re-loading from a save does not fix it (even loading from a save before the bug). Quitting the program completely and reloading the same old save does restore the sound though.
Loading a save made after the bug occurs kicks the bug in again.

I have the same exact issue, in the same place, in the eavesdropping mission. Worked fine the first time, reloaded the mission and surprise, no more sounds!

I tried everything, switching devices, nothing works. Can't set device to 2.1, it's a pair of Bose Companion 5's, the only option is 5.1.

Please fix this bug, I can't play the game anymore!

I have had issues with audio and the only way to fix it for me was to go into the windows control panel, locate the audio out that I'm using (headphone), disable the device, wait a minute, re-enable the device, then restart the game. Don't know if it will work for you.

Not sure why this works...I think someone said that for some reason if you have multiple audio out enabled that the game might switch where its sending the output.

Some tests I made so far:
- if I start the game and load the sneaking save there are no voices.
- if I start the game and load the sneaking save then load another save there are voices.
- if I start the game and load a save from the beginning of the chapter then load the sneaking save, there are voices.
- if I start the game and load the sneaking save (no voices) and finish the quest and leave the area (autosave occurs) then restart the game and load the last save, there are voices
- if I start the game and load the fighting club save (same area), there are voices
Further testing may reveal the precise trigger, nevertheless I think it is quest related.

THAT would be worth a try! I am desperately trying to get the voices back.. You say finishing the quest AND leaving the area might do the trick ? Well, I will try this , although playing without those beautiful dirty atmospheric voiceovers is ... saddening ;)
Thanks for your testing, I will report back if it workes out.