The sound travels through helium significantly more quickly than through air when a flute is filled with it as opposed to air since helium is substantially less thick than air, sound moves through it more quickly.
The frequency of a sound wave in which is the quantity of vibrations per second and it determines the pitch of a sound. Since helium has a higher sound speed than air, the sound waves created by the flute will move more quickly and at a higher frequency, producing a higher-pitched sound.