See https://forums.plex.tv/discussion/181255/rasplex-guide-to-audio-settings for a audio settings guide.
Before RasPlex 0.6.0 the number of channels for PCM audio was always forced to only two channels, this is why it worked in earlier versions of RasPlex, for passthrough the AC-3/DTS stream channels was used.
In RasPlex 0.6.0 and newer the number of channels in settings actually affects PCM audio channels and should match what your setup supports, most TVs unfortunately only support stereo PCM passthrough and some support AC-3/DTS passthrough.
I would suggest you set channels to 2.0 and test the different capable receiver settings.
