Dolby Mono:

This program features a mono soundtrack encoded to a AC-3 bitstream. When played through Dolby Digital equipment only sound from the center speaker will be heard.   There is also two channel mono where the same information is passed to both the front left and front right speakers to give a broader feel to the mono sound field. This is actually very common, to use the two front speakers to deliver mono sound. You may want to by pass the digital processing and go to one of the emulation modes of the receiver to provide a fuller sound and utilize the rear speakers and sub woofer.