Why does the act of inhaling helium change your voice?

When we talk air passes out from our lungs through our vocal chords and is caused to vibrate and you hear the words. When you mix helium with air the resulting mix is less dense so as you breathe out the timbre of your voice is altered. The speed of sound in a less dense helium/air mix is higher than in just air.  You are increasing the speed of sound of your voice when breathing out this less dense mixture and this increase in speed will affect the frequencies that make up your voice. If the frequencies are affected the timbre is affected too. A more dense gas may have a very wierd affect.
Strictly speaking although most people say it is the pitch that changes, it is the actually the 'timbre' which is changed, as your vocal chords produce the same shapes and therefore the same pitch.