Question

WHEN THE SOUND REACHES THE EARDRUM, IT CAN PUSH THE EARDRUM INWARDS BUT HOW CAN SOUND SET IT VIBRATING

Answer 1

It is important to note that basically sound is a form of energy. When the sound waves hits the surface of a material it imparts some energy. The molecules of the material would receive that energy and start to vibrate more vigorously, the material as a whole vibrates. It in turn would produce some sound as all vibrating do.

In this case we can substitute the material for an ear drum and the same will happen, it vibrates on contact with sound.