When you speak after breathing helium, in which the speed of sound is much greater than in air, your voice sounds quite different. The frequencies emitted by your vocal cords do not change since they are determined by the mass and tension of your vocal cords. So what does change when your vocal tract is filled with helium rather than air?

Answer: Increase in velocity

Explanation:

It is true that that frequencies do not change when you breathe in helium instead of air.

The reason voice sounds different lies in the nature of helium that it's density is lower. This leads to it travelling faster to the vocal cords as compared to the regular air. It is not changing the frequency rather by travelling faster to vocal cords it is affecting the quality of sound and result in changing of the resonant pulses or sounds of your vocal tract as it becomes more responding to the sounds with high frequency.

We have v = fλ

Since frequency is not changing and wavelength also remains same this implies the velocity is changing resulting in producing voices that sounds different.