After flying multiple missions for several days, a Covey returned to base and landed with his gear up.  A quick listen to the tower tape confirmed the tower had advised him that his gear was not visible; the Covey asked for a repeat transmission from the tower; the tower repeated the advisory.  The gear-up landing was already fodder for the accident investigation board, however.   Listening closely to the tower tape, the gear warning horn was discernable in the background.  The Covey stated he could not hear the warning horn because of the excessive background noise in the cockpit.  Could it have been the background noise was excessive because of the warning horn?   Nah!