What gas makes your voice deeper
Sulfer hexafluoride is a gas that's 6 times heavier than air, which will cause the vocal cord cords to vibrate more slowly. This in turn decreases the frequency of them, lowering the pitch of one's voice.
This works in the same, but opposite way that helium gas does. Helium is about 6 times lighter than air, and allows the vocal cords to vibrate faster, thus raising the frequency of sound created by them.
Sulfer hexafluoride has many commercial uses, and there is an increasing demand for it, however even though it's inert and non-toxic it's also a "potent greenhouse gas," which means that is can be a major contributor to climate change.
Caution: Sulfer Hexafluoride should not be used without proper training. Since it is so much heavier than air, it cannot be easily cleared from the lungs, and could suffocate someone that's inhaled it. Additionally, it is described by Airgas, a company that sells it, as a substance that may cause damage to the upper respiratory tract.