what gas makes your voice lower
Pure helium is a medium that allows sound to pass through it more quickly than air, so it makes your voice sound higher pitched (the mechanics are similar to speeding up a video and hearing the sound pitch up). Some gases will do the opposite; most commonly, sulfur hexaflouride, which slows down sound waves.
A less familiar, and more dangerous trick, is to repeat the process using a very dense gasâsulfur hexafluoride. Just like low-density helium displaces the air and creates a high-pitched voice, sulfur hexaflouride also displaces regular air but creates a very low-pitched voice thanks to its high-density.