Normally, if you have a motherboard with built-in multi-channel audio HD audio codec chip support (through related mini-jacks, via the onboard DAC from the audio chip) and you have an appropriate external 5.1 audio speaker system, you'd just connect the speakers to the associated mini-jacks. You'd then go into "playback devices" (from a right-click on the speaker icon in system tray), select "speakers" to configure both properties (for "supported formats" tab) and configuration (i.e. 5.1).
This approach uses something like the Realtek chips and associated HD Audio Manager software/drivers to provide multi-channel output directly from your motherboard's speaker mini-jacks to your speaker system. It supports any media player program (including WMC, set to 5.1 which is what HDTV is broadcast with) which supports multi-channel audio.
And "playback devices" specifies "speakers".
Alternatively, if you have an external AVR which manages your external speaker system, you need to route digital audio from the computer to the AVR. This is typically done in either of two ways: (1) optical via S/PDIF output from the discrete sound card in your PC (or motherboard S/PDIF connector if present), or (2) HDMI via the video card's HDMI output, so that the HDMI cable is "routed" through the AVR where audio gets "dropped off" and then out of the AVR with a second HDMI cable to your HDTV display device for video.
Or, you can just send audio-only out HDMI from the video card (using the video card's HDMI audio driver), and separate video out via DVI from the video card to your monitor.
Again, you use "playback devices" to designate where/how the digital audio gets from your PC's possibly multiple digital audio outputs (optical or HDMI) to the external sound system. And then in Properties -> supported formats for the selected output device you check the multi-channel formats so that the original multi-channel DD5.1 is sent from the HDTV program to your AVR for decoding and delivery to the attached external speakers.
Since HDTV is only DD5.1, and since optical cables support DD5.1, the simplest and still 100% acceptable way to get multi-channel audio to your sound system while also providing HDTV video to your monitor or TV is to use the optical connection for audio (with the multi-channel "supported formats" checked) to your AVR. Then you're free to use either DVI or HDMI for video, to a connected monitor or HDTV.
WMC doesn't need anything other than 2-channel stereo or 5.1 audio, since that's all that HDTV supports.
The rest of your audio/video system configuration is up to you and your hardware, either built into the motherboard or available through external expansion cards and their available audio/video connectors.
I myself use the optical cable approach for multi-channel audio (as my ASUS P8Z77-V Pro board has S/PDIF optical output available) to send digital multi-channel audio directly to an optical input on my Yamaha AVR. The AVR does the decoding and sends multi-channel sound to the connected speakers. I don't use the analog speaker outputs from the motherboard. "Realtek optical" is selected in "playback devices", configured to support all multi-channel formats.
Video from the PC (HDTV from WMC or from any other windows function or program) goes to my dual 24" LCD monitors via my dual-DVI ATI HD5870 video card.
All audio from my (1) Ceton InfiniTV4 4-tuner cablecard-enabled TV tuner card, and (2) Hauppauge HVR-2250 2-tuner OTA/ATSC TV tuner card, as picked up by WMC, is simply routed directly to the S/PDIF optical audio output along with ALL OTHER AUDIO from Windows.