[Aux_]

I do not quite understand,
my AV Receiver shows stereo on Live TV, but when I use OMXPlayer, he shows Dolby Digital on display, why that?

that's likely because omxplayer still allows for passthrough on live streams, while videoplayer does not. Try setting the number of speakers in System -> Audio settings to the number of your physical speakers (5.1 or 7.1, whatever you have). After that, you should get 5.1 PCM. The limit to 2 speakers for passthrough is only needed for SPDIF connections AFAIK. 

if I turn on in the audio settings PLL adjustment, then everything appears correctly on the display of AV Receiver. It shows PLII C when 2.0 dolby, Dolby Digital when 5.1 dolby. When I disable PLL adjustment it only shows stereo or "multi channel stereo".