They can definitely sense vocal vibrations if nothing else, and also distinguish between different vocal pitches.
Proof of this I personally witnessed, here is the story.

For a period of about 3 years, I used to go and feed 13 African House Snakes, that a woman had.
I did this every 10 days or so. She could not bear to feed them.

While waiting for the thawing process to complete, we would normally sit in the snake room and talk about whatever, having a beer or coldrink.
After about 4 to 5 minutes of talking you would see heads start to pop out of hide boxes. This never occurred when she and her children were talking in the snake room.

So we did a test, with me going to feed one day and not making a sound for about 15 minutes or so, I then started talking and lo and behold, after a couple of minutes, the heads started popping out of the hide boxes. The only thing I can put it down to is that they associated my Vocal Vibration and Pitch, with food.