sulfur hexafluoride

A very dense, inert, non-toxic gas that is much heavier than air and used in the electrical industry as an insulating gas. If you inhale it, it would make your voice sound like Satan, i.e. really deep. This is because sound travels through this gas slower than it does through air.

When the science teacher demonstrated how sound travels differently through different gasses by inhaling helium and sulfur hexafluoride respectfully, he began to talk like a chipmunk and then like Darth Vader.

Justin Bieber inhaling sulfur hexafluoride would probably result in his voice sounding like a normal maturing teenage boy's voice, rather than like a little girl's.