[Aux_]

I use neither PCM cable nor SPDIF but HDMI and I believe that it had worked in earlier versions of v18.

you'll get 5.1 or 7.1 PCM via your HDMI connection - there is no special cable for that. Only SPDIF is limited to two audio channels and thus requires AC3 or DTS passthrough to get surround sound to the AVR (5.1 audio is encoded onto the two available audio channels of SPIDF). So when your AVR is displaying PCM 5.1 when watching liveTV, all is fine and you won't have any quality loss over any AC3 or DTS passthrough. It doesn't matter if Kodi is decoding AC3 into 5.1 PCM or if it's the AVR that does it - the result will always be the same digital audio signal (unless the AVR is applying custom post processing).
Passthrough was mostly relevant for SPIDF connections, since it was the only way to get "true" surround sound to your AVR (ProLogic just wasn't it). With HDMI connections, passthrough is basically obsolete these days, except for Dolby ATMOS, for which the AVR requires the metadata of the original audio stream to correctly place the audio signals in the virtual space, since the metadata is lost/gone on decoded PCM streams.  

thanks for the explanation @da-anda 

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?

My AV receiver see signature