Question

why we can not see sound?

Answer 1

Sound waves are propagated through the air molecules. You detect light because each photon carries enough energy to cause a chemical transition in your retina. While sound propagation happens through the vibration carried forward by each air molecule with in the picture. The energy in a sound wave is too widely dispersed to cause the same chemical transitions in your retina. So its not possible to see sound waves though you can see the sound waves physical effect that form ripples on the surface.