When you pluck the rubber bands, it vibrates the inside of the box making a sound from the box. If you pluck a different kind of string, the sound will be different because of the length or weight of the rubber band.
When you blow into the top of the flute, the air will vibrate and come out the other side but since there is a small hole in the flute, the air has two places to come out so the vibrations inside are coming out different sides so the sound is different
When you blow through the holes in the harmonica the air molecules travel through and come out the other end so we can hear it. The longer the straw is the higher the pitch is that is because the sound waves are high so the sound is high too.