The channels on DirecTV are whatever the network has sent to DirecTV, most today are in stereo. DirecTV even has over 20 channels that broadcast Dolby Digital.
After upgrading to DirecTV (from AT&T), I've noticed much improved dynamic range and wider channel separation. And as Roger has pointed out, many premium channels (HBO, ShowTime, Starz, etc.) broadcast a good number of programmings in 5.1-channel Dolby Digital each night.
Although DirecTV's video quality has suffered due to higher compression, it still sounds great (for TV anyway).