Why does helium make your voice squeaky?

The common explanation is wrong in an interesting way. Helium does not raise the pitch of your voice, because pitch is set by how fast your vocal folds vibrate, and that rate does not change with the gas around them. What changes is timbre. Sound travels roughly 965 meters per second in helium against about 343 in air, and the resonant frequencies of the vocal tract, called formants, scale with the speed of sound. Boosting those resonances shifts which harmonics get amplified, producing the thin, ducklike quality. A spectrogram shows the fundamental frequency essentially unchanged while the formants jump.

The reverse demonstration makes it obvious. Inhaling sulfur hexafluoride, a gas far denser than air, slows sound and drops the formants, producing an exaggeratedly deep voice while the vocal folds again vibrate normally. Worth stating the safety side plainly, since this is a party trick people repeat: helium displaces oxygen, and inhaling it from a balloon can cause dizziness or fainting, while inhaling directly from a pressurized tank has killed people through lung rupture and gas embolism. Helium is also a genuinely finite resource, produced by radioactive decay over geological time and largely irrecoverable once released, and it is essential for MRI machines and scientific cryogenics, which makes the balloon supply a real if minor concern.