I could not get my home theater AV receiver to play in 5.1 surround using Chromecast. After some investigation into the user manuals of my TV and audio receiver, I have come to the conclusion that it is simply impossible with my setup and Chromecast.
The Samsung LN55B640R3FXZA 55" TV will only output 2 channel stereo via the digital optical out when receiving from HDMI.
My Yamaha HTR-6130 AV receiver will only pass through audio from HDMI inputs to the HDMI output without intercepting the audio to supply the receiver itself. You have to connect a separate digital optical or SPDIF cable from the source unit to get 5.1 to work.
So, since the TV will not transmit the surround to the receiver and the ChromeCast has no independent digital audio out I simply cannot play back surround sound with my system. I thought this might help save some people some time from trying a dozen different transcoding schemes to get surround to work when there is a possibility that their system is not capable of doing it.
I think I will take the ChromeCast back and look for a Blu-Ray Player with Plex support instead. I'm looking at the Samsung BD-H6500. I found a device capable of stripping the audio from the HDMI cable, but it is $100 and the Blu-Ray player is $150 and can also play DVDs and Blu Discs, which ChromeCast cannot do.
I hope this helps someone because I spent quite a few hours transcoding with HandBrake with different setups thinking I was not doing it right when it was my AV setup that was the shortfall of accomplishing surround sound with my stream.