Sound travels faster in helium than in air because helium atoms, with atomic mass 4, are lighter than nitrogen and oxygen molecules, which are molecular mass 14 and 16 respectively.
In the human voice, as in all wind instruments, the sound is produced as a standing wave in a column of gas, normally air.
A sound wave’s frequency multiplied by its wavelength is equal to the speed of sound.
The wavelength is fixed by the shape of the mouth, nose and throat so, if the speed of sound increases, the frequency must do the same.
Once sound leaves the mouth its frequency is fixed, so the sound arrives to you at the same pitch as it left the speaker.
Imagine a rollercoaster ride.
The car speeds up and slows down as it goes around the track, but all cars follow exactly the same pattern.
If one sets out every 30 seconds, they will reach the end at the same rate, whatever happens in between.
In stringed instruments, the pitch depends on the length, thickness and tension of the string, so the instrument is unaffected by the composition of the air.
Releasing helium in the middle of an orchestra would therefore create havoc.
The wind and brass would rise in pitch, while the pitch of the strings and percussion would remain more or less the same.
In the Song of the White Horse by David Bedford, the lead soprano is required to breathe in helium to reach the extremely high top note.