What gas makes your voice deeper?

Answer 1

The speed of sound varies with the density of whatever it travels through. Something less dense than air means a faster relative speed of sound, something more dense means a slower relative speed of sound.

The slower sound moves, the lower we perceive it's pitch, the faster the speed, the higher the perceived pitch.

Vocal chords stretch and tighten to make sounds of different pitches. If you replace the air (Nitrogen-Oxygen mix) with something LESS dense like Helium (1/6th the density of air) then the sounds made by the vocal chords working exactly the same as they did in air, will have a higher apparent pitch because the sonds they make move faster through helium than they do through air. Please note, the vocal chords are NOT moving any faster than they do in air.

So with something more dense than air (like xenon about 4 times more dense, or even better Sulfur Hexafluoride about 16 times more dense than air and 40% more dense than water!) the sound will have an apparently lower pitch, because the sound made by the vocal chords will move more slowly through the denser gas and so have a LOWER perceived pitch.

Answer 2

Sulfur Hexafluoride