Sound waves are normally mechanical wave i want to know whether the sound wave bends if it bends does it undergo diffraction just like light wave?
Sure sound waves diffract. That's why two people can have a conversation, even if there's an object in between them.
When I teach this stuff, I often walk out of the classroom, to a place where the students can't see me, just to show that they can still hear me. That illustrates that sound waves diffract, and moreover that they diffract much more than light waves under these circumstances. That's because the wavelength of sound waves is comparable to the other length scales in the situation, whereas the wavelength of light waves is much shorter.
Sound waves do, indeed, bend. In an underwater thermal gradient the sonar waves can bend enough to create blind spots, so that you can not "see" a target that would ordinarily be well within sonar range.
The other example is the difference in how well sound carries up/down wind. Wind velocity increases with height (at least in the near surface boundary layer), this shear means that soundwaves traveling upwind are diffracted upwrds, but waves travelling downwind are diffracted downward, so that in the downwind direction the sound can travel in a channel near the ground.