I was playing the town segment in Thief deadly Shadows and went on a killing spree for fun.
Below the screams and the guards shouting, I started hearing this really creepy distorted voice saying something I couldn't comprehend. What in the F was that?

If you were in Stonemarket Proper and haven't let the captive Kurshok out of his cell, it might be his mutterings.

you may be right...hmm...

If you heard lots of clicking sounds, gravelly yoda like voice then it was a Kurshock in the holding cell. Again as VanishedOne mentioned you would have had to have been in Stonemarket Proper.