[Maconi]

It works for 5.1 with everything in my setup (even my Wii).

But the same HTPC with Windows on it only sees my TV's audio (2.0) and not my Receiver's audio (5.1). However, it still plays that 2.0 through my Receiver's speakers.

It's Realtek integrated audio (ALC892). Any advice for forcing 5.1?

EDIT: Scratch that, I answered my own question. It turns out my TV will push Analogue 5.1 through to the Reciever but if it is Digital 5.1 it only outputs 2.0 (which is why my Wii which is connected with Component cables works in 5.1 but not my HTPC which is connected with HDMI).

✎DIGITAL AUDIO OUT (OPTICAL)

• 5.1 CH (channel) audio is available when the TV is connected to an external device supporting 5.1 CH.
  
• When the receiver (home theater) is set to on, you can hear sound output from the TV’s optical jack. When the
  TV is receiving a DTV signal, the TV will send 5.1 CH sound to the home theater receiver. When the source is
  a digital component such as a DVD / Blu-ray player / cable box / STB (Set-Top-Box) satellite receiver and is
  connected to the TV via HDMI, only 2 CH audio will be heard from the home theater receiver. If you want to
  hear 5.1 CH audio, connect the digital audio out jack from your DVD / Blu-ray player / cable box / STB satellite
  receiver directly to an amplifier or home theater.

[StevenSeagull]

I've tried sending 5.1 DTS and AC to my tv rhrough hdmi and coax back to my reciever.
Turns out that my tv would only pass 5.1 AC3 but no dts to reciever presumably to prevent some damn copy protection or something similar.

[nevcairiel]

It probably just doesn't support DTS.

The easiest would of course be to connect to the Receiver via HDMI and from there to the TV, but since you did it this way, i am guessing your receivers don't do HDMI?