Summary: I use the concepts explored in Sound Waves to explain the effect of inhaling helium on frequency and reception. The idea of the speed of sound in relation to density is the main underlying idea.

https://blogs.ubc.ca/phys101/files/gravity_forms/1-72f8f8fec225ededbd95c074100f3d9f/2015/02/sound-waves-lo-jeff-yu.jpg