Sound travels almost 3 times as fast in helium gas than in normal air because helium is less dense. Contrary to common perception, helium does not make the voice high-pitched, because voice pitch is determined by the vibration frequency of the vocal chords generating the sound. So, while the fundamental pitch of the voice is actually unchanged, what helium does is change the timbre of the vocal sound compared to speech produced in normal air.
The vocal tract, nasal cavities and mouth create acoustic resonances, known as formants, from the sound generated by the vocal cords. Because the sound waves move faster in helium-laden breath, the stronger resonances shift towards higher frequencies and drastically change the overall tone quality of the emitted voice.