The accumulation of electric charges has to be great enough to overcome the insulating properties of air. When this happens, a stream of negative charges pours down towards a high point where positive charges have clustered due to the pull of the thunderhead. The connection is made and the protons rush up to meet the electrons. It is at that point that we see lightning and hear thunder. A bolt of lightning heats the air along its path causing it to expand rapidly.
Thunder is the sound caused by rapidly expanding air.