Why does helium make your voice sound funny?
The way your voice works is that your vocal chords vibrate and make a series of different frequencies. Your throat and mouth act as a resonant cavity. This is a bit like a tray of water, you can vibrate it at lots of different frequencies and not much will happen, but if you vibrate it at the same speed as a wave will travel up and down it, the wave will get stronger and stronger. Similarly if you vibrate your throat at the right speed the vibration will get stronger and stronger so you throat will amplify some of the frequencies your vocal chords are making (by changing the shape of your mouth you can alter which ones are strengthened which is how you make different vowel sounds). Sound travels faster in helium so the vibrations will get from your vocal chords to your mouth quicker so your throat will resonate at a higher frequency and hence a higher pitch.
WARNING - There is no oxygen in a helium balloon and your body has no way of detecting that it is short on oxygen so you will feel fine until you fall unconconcious. So it is recommended you don't try and replicate this. If you must have a go, don't take deep breaths of helium or more than one breath of helium every few minutes.