Why does helium make your voice high-pitched?

Your voice’s pitch depends on the vibration speed of your vocal cords, which does not change when you inhale helium. What does change is the resonant frequencies of your vocal tract. Sound travels at about 343 m/s in air but about 970 m/s in helium because helium molecules are much lighter. The faster speed shifts the resonances of the throat and mouth upward, amplifying higher overtones and making the voice sound squeaky. The opposite effect happens with a dense gas like sulphur hexafluoride, which makes the voice deeper. Inhaling helium is generally safe in small amounts, but inhaling it from pressurised tanks can be dangerous.