If you are running the hdmi from the cable box, to the receiver, then to the tv, the tv's sound options do nothing, as the sound is hitting the receiver first. Therefore, you would need to look into the cable box or the receiver options to set Dolby, but most likely the cable box. And FYI, PCM is uncompressed sound, which is better than dolby (compressed). If you are seeing PCM 5.1, that is the best you can get, better than dolby 5.1. If you are only seeing PCM 2 channel, that is an issue with a setting somewhere.