Many people, on hearing the voice of someone who has breathed helium, believe that the person's speech pitch has increased.
Warning - Breathing helium can be very dangerous.
A cavity will have certain resonant frequencies. These frequencies depend on the shape and size of the cavity and on the velocity of sound within the cavity. Human vocal cords vibrate impulsively (pulse rate is the voice fundamental) in the vocal tract, generating a range of frequencies above that fundamental. The vocal tract and cavities enhances various frequency components imparting the recognizable voice spectrum.
The velocity of sound in helium is more than twice that in air. The characteristic resonant frequencies of the vocal tract cavities will be raised in that ratio. The mechanical resonant frequency of any solid or fleshy tract component will not be altered by helium, but the result of the higher resonance frequency of the several vocal tract cavities is to alter substantially the relative amplitudes of the voice spectrum components and harmonics thus leading to a significant voice timbre change and also an apparent pitch change.
Source: Acoustics FAQ (website), http://www.faqs.org/faqs/physics-faq/acoustics/