In the channel in which discharge current flows dominate high temperature and high pressure which contain "conductive plasma ". This condition lasts while current flows through channel. The current is serried by forces of its magnetic field on a small cross-section. When current flow is stopped the pressure of plasma starts breaks free and spreads radially.
This air pressure affects our ears and we say that we hear the thunder. In the past people didn't understand phenomenon of thunder and lightning and they thought that it was sign from Gods. For example, the old German god Donar represented with the hammer in his hands was incarnation of thunder.

Pressure wave moves. Highest pressure is in the area close to the lightning, only few centimeters far from channel of lightning, then getting lower on some distance. Thunder, which is acoustical effect of this pressure, can be heard on some distance of thunder impact like detonation with rumble. If we are far from the impact, we hear some noise. On distance of 10 km we don't hear anything. If we are very close to lightning impact we could lose every feeling about this act. This is happens because of the high pressure which rule in this area.