He (and SF6)

As the video demonstrates, the amusement potential of helium remains undiscovered within the latex of the pretty balloons at your little sister's birthday. The symbol "He" might as well stand for "Hours of Entertainment". But how does this rising gas bring your voice along for the ride?

The essential characteristic of helium that enables this effect is its lower density, specifically compared to that of normal air. Imagine bringing an empty, sealed water bottle to the bottom of a pool. Intuitively, we all know that the bottle will rapidly rise to the surface. This is because the bottle weighs less than the medium (water in this case) that the bottle has displaced, or taken the place of. Similarly, helium is less dense than plain air and therefore weighs less than it in the same area, causing the balloon to experience buoyancy, albeit in a medium of air.

Another result of lower density, apart from floating felines, is the way in which sound moves through it. The speed of sound varies with the square root of the density of the gas. In layman's terms, the denser the gas, the slower the sound. In helium, sound moves 2.7 times as fast as in plain air, and with sulfur hexafluoride 0.44 times as fast. With a faster speed comes a higher frequency and pitch, and vice versa. 

In honor of The Martian, I found that the composition of Mars' atmosphere is mostly carbon dioxide, which is denser than "Earth air". That would mean that if I somehow breathed the poisonous air and lived, I would speak in a lower pitch. Cool, huh?

Sources:

Landsberg, Randy. "Balloon Laboratory." (2002): n. pag. Kavli Institute for Cosmological Research. University of Chicago. Web. 22 Oct. 2015.

Sprott, Clint. "Sound." Sprott's Gateway. University of Wisconsin - Madison, n.d. Web. 22 Oct. 2015.

Your Voice on Mars. Perf. Dr. Bruce Betts. The Planetary Society. N.p., 23 Oct. 2014. Web. 26 Oct. 2015.