Sound from a flute and sound from a harmonica played by passing air through musical instrument, at same time and same distance from ear and traveling through same medium air, they differ in sounds due to difference in
Pitch
Pitch of sound is how brain interprets the frequency of the emitted sound. In harmonica, vibration of particles is more at the source, and hence more frequency. Therefore, it has higher pitch. Thus, harmonica sound appears different than sound of flute