The reason your voice sounds higher when you have helium in your lungs is because of density. Density is the mass of an object divided by its volume. Mass and weight aren't quite the same thing, but if you're not familiar with the term it's close enough. When you have a lot of stuff (mass) packed into a small area (volume), it has a high density. When you have less stuff per area, the density is low. Low density things float on top of high density things. You know that a helium balloon rises, right? That's because the density of helium is less than the density of air (which is about 80% nitrogen).

What does this have to do with a squeaky voice? Well, things move more slowly through denser material. That's why it's hard to run when you're in a swimming pool. Sound waves traveling in low-density helium move about twice as fast as sound waves traveling through dense air. Faster moving sound waves have a higher pitch.

If you were to breathe in a denser gas, how do you think that would affect your voice? What do you think a whale's call would be like if it were calling on land?

There's a fun video of a college professor demonstrating how different density gases influence pitch:

Keep in mind that many gases are toxic to breathe, so don't experiment on this yourself.

Your vocal cords can flap more easily through light helium than through heavier air.Easier flapping means faster flapping, which leads to higher frequencies. If you wanted a deeper voice, use argon, which is even heavier. In either case, just be sure you take deep breaths of fresh air afterward. You need oxygen. :-)

Good question! The pitch of your voice (or how high or low your voice sounds) is determined by the speed at which air passes over your vocal cords. The faster the air travels over your cords, the higher your voice sounds. The air we normally breathe is approximately 78% Nitrogen, 21% Oxygen, and rest Argon, Carbon Dioxide and trace amounts of other gasses

Since the concentrations of air normally remains the same, the pitch of our voice is unaltered. When we inhale Helium from a balloon and talk while exhaling, our voice becomes high pitched because Helium is a very light gas and therefore is traveling up (towards the upper atmosphere) very fast. Since Helium is traveling fast over our vocal cords, we get a high pitch to our voice.

I think this is because the sounds we make involve vibration of the vocal cords...this vibration set up sound wave that travel through air. Now the pitch is related to the sound vibrations that travel through air; the density of the air affects the pitch, so if Helium replaced air, due to the different density of He relative to air, the pitch would be different.

Helium gas is lighter than air, which contains mostly nitrogen, oxygen and argon gases. As helium moves out through our lungs and we vibrate the helium with our voice, it doesn't vibrate in the same way that air would. There's a great and detailed answer to this question here:

Things like our vocal chords vibrate at different frequencies in different fluids. Picture stretching a spring and watching it oscillate in air vs water.

This answer to your question has to do with physics and human perception.Sound travels faster in helium air. Therefore, humans perceive higher pitches from voices through helium air.