I have a VMS1100 STB and I have the audio set to surround. When I connect my STB to my TV I use the HDMI cable. From my TV I then connect to my home theater reciever with the optical cable. My receiver only plays Linear PCM. If I directly connect the optical connection from the STB to the home theater receiver then I receive a DD 5.1 signal. I need to connect through my TV so I can free up a digital connection on my home theater reciever. I have two one optical and one coaxial. The coax is being used by the dvd player. I just bought a Chromecast and I need to have a digital signal out of my TV to connect to the home theater receiver. If I run the STB through the TV via HDMI and then to the home theater receiver I lose my DD 5.1 signal but then I can use the Chromecast with DD 5.1. Is there any way to change or update the STB to change this? When I had Comcast I had no problem with the audio passing through the STB to the TV to the home theater.
Is the Linear PCM in stereo or surround ? If it's in stereo (2 channels only), then maybe your TV is the cause, not the STB. Have you checked all the TV's Audio settings ? Does your TV output 5.1 channel surround through it's optical jack ? What make and model is your AVR ?
Have you tried connecting the STB out to the receiver in via HDMI, then going out of the receiver via HDMI to the TV in?
You should then get your 5.1 audio to the receiver via HDMI, and then you can disconnect the optical cable from the STB to the receiver to free up that input on the receiver so that you can use it for the TV's optical output.
Also, the thread below might be of interest to you: