Why does the voice change with helium?

 

When you inhale ambient air, your vocal cords vibrate in "normal" mode. 

 

However, helium has a lower density, around 7 times lower than air. As a result, the vocal cords are less strained and vibrate at a higher frequency. The voice becomes higher and more squeaky. 

 

And if you inhale a heavy gas, such as sulfur fluoride, which is five times heavier than air, even a 7-year-old girl will start to speak in a bass.