I have Raspberry Pi connected to 7.1 surround system using older amp with external 7.1 HDMI to analog extractor.
Here is the issue I'm facing when trying to play DTS-HD Master or Dolby TrueHD 5.1 audio on my system. The sound is mapped to 5.1 speakers in my 7.1 system leaving rear left and right speakers silent. If I choose a lower quality sound track DD or DTS 5.1 and optical SPDIF cable the receiver will map and link Surround Left with Rear Left speaker and the same on the right side.
Looking for a software solution to play high quality DTS-HD and TrueHD 5.1 source on 7.1 surround bridging side and rear speakers.
My other option is to use hardware RCA "Y" splitters and bridge side and rear channels by jumpers for 5.1 source and removing jumpers restoring original cable configuration for 7.1. Very annoying specially when you need to crawl behind the cabinet to get to the rear plugs.
There is an option under Audio Output Configuration -> "Best Match - Output properties are set to always be as close a match to the source properties as possible".
This looks like a perfect spot to include speaker matching 5.1 to 7.1 and 7.1 to 5.1 once you declare the "Number of Channels" at the top of the configuration page.
I'm assuming that in the opposite case having 5.1 surround speakers and trying to play 7.1 source you would miss the rear channel sounds all together using RPi and LPCM decoding.