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I have a Sapphire HD Radeon 4650 hooked up via HDMI to my sound system. Everything works if I playback DVD/Blu-ray media (with proper digital decoding) - properly redirected to all 5.1 speakers. All my speakers are working - my configuration for the HDMI audio is setup for 5.1 and I can hear sounds on every channel when I do the speaker test. However, if I watch Hulu, Netflix, or playback Mp3's... I only seem to get audio on 2 front channels + subwoofer.

I have media player classic, and VLC... I can see ffdshow filters when I run media player classic and there's a lot of options for sound settings. There are also a fair number of sound setting withing the volume control panel for the Realtek HDMI ATI audio settings (one of which is a Speaker Fill... which does in turn pump out to all speakers... but sounds like crap). Amidst all these settings (ffdshow, media player classic, sound controls)... is there a way I can upmix properly to 5.1 when (for Hulu, Netflix, playing videos/mp3's off my PC)?

What's the "proper" way to mix 2 channels up to 5(.1)?

It's playing stereo when you're feeding it stereo. There are lots of different ways to mix stereo up to 5.1. Whatever receiver you're sending it to probably has ways to do it. Your sound drivers might have ways to do it. FFDshow audio has ways to do it. AC3Filter has ways to do it. There are lots of algorithms and tools to mix to 5.1, but not really a "proper" one unless you're doing dolby pro logic with a dolby surround source. There's no magic bullet to "properly" mix 2 channels into 5 - it's down to preference.

What mattsteg said. If you just want to duplicate the front left and right speakers to the rears, that's easily done in ffdshow's mixer options.

Something's screwy with my settings or whatever for ffdshow. I can only hear resampling to all the speakers in 5.1 if I set it to quadro or 6 or 7 channels (and that doesn't sound right). When I set it to 5 channels (which it should be set to), I hear a noise that sounds sort of like a tape-deck player running past the end of the reel... is this digital data trying to be decoded? Help would be much appreciated.

What are you connected to? Doing your upmixing in your receiver might be a good option.

I use my receiver to upconvert stereo audio to 5.1, and the results are quite decent. I had less luck when using software on the machine itself to do this.
I'd suggest letting your receiver do the job.

Doesn't seem like there's a setting on my receiver to take what's coming from the PC (HDMI) and upmix it to the other speakers - I've tried iTunes + every sound field setting on my receiver and can't get any sound upmixing out of the rear channels. ffdshow audio works great for me when i have the options set to 6 channels and also enable DTS and Dolby (when using MPClassic, it know when the file audio format is digital and sends this to my receiver instead of upmixed audio - which it sends for MP3's). So, with a combination of media player classic and VLC, I can play any video (or mp3) file that's originally only 2 channels and have it upmixed to 5.1.

Two major problems however seem to be playing content through a webbrowser (Hulu, Netflix), and playing song via iTunes. Playback of these items seems only to result in 2.1 audio from my receiver. Again, it may be because the connection is HDMI and the receiver is unable to upmix (no idea... I have a sony HT-SS360). For TV watching, the audio all comes in 5.1 because the OTA broadcast audio is pumped via toslink optical audio and receiver's set to Dolby Prologic II (which seems to work properly for sending the right sounds to each channel in true dolby 5.1). For blu-rays, everything's true bitstream and the content plays back perfectly in 5.1

Only problem is with the damn PC. If there were a way to have ffdshow intercept all audio (itunes, webbrowser), that would solve my problem, as ffdshow does a great job... however, I've yet to find a practical solution such as this.

Any ideas?

AFAIK not all apps are "media centric" enough to allow you to select the audio path to go through ffdshow/whatever, so they would just go to "default". The problem on the PC is that, at least on my machine with the HD 4550, it is configured to either DD/DTS passthrough, or 5.1 LPCM. When my receiver sees 5.1 LPCM, it knows not to touch the signal even if only 2.1 of them contain real sound. So you don't get the upmix.

You will have to manually change the speaker setup to 2 speakers, then my receiver automatically upmixes with Dolby Pro Logic, even if the signal is not properly encoded (it tries to the best of its ability). Speaker fill is just going to duplicate the sound on all speakers.

Flying Fox, I think I'm encountering the same issue as you (regarding the seeing 5.1 LPCM and no upmixing regardless what the receiver's sound-field is set to)... didn't occur to me that the reason the Dolby Prologic II sound-field setting not upmixing could be because I have windows set to 5.1 speaker configuration. I'll take a look at setting it to 2 speakers tomorrow and see if I can get that 5.1 upmixing via the receiver doing the work.

One problem I'm wondering is - if I set the speaker setup to 2 within windows (and set my receiver to D PL II upmixing)... will this mess up the ability of 5.1 digital audio (true dolby digital) from pumping from my PC to the sound system? or will it see the digital signal (and decode) while at other times, it'll see 2 channel LPCM (and upmix, since it'll be set to D PL II)? I'll give it a try later (so as not to get evicted).

Aright - this one's got me baffled.

I've tried changing the windows speaker settings on my HDMI sound profile to 2 speakers... and I still can't get receiver-end 5.1 upmixing despite cycling through all the sound-field options. I'm not quite sure how something like Ffdshow is able to do 5.1 upmixing to the center/rear speakers if what was mentioned is "not touching 5.1 LPCM signal."

EDIT:

Only way I can get upmixing is if I go into the sound enhancement options in the volume properties and enable speaker fill (and this sounds crappy - like the rear speakers are under water). I would think the receiver would be able to do 5.1 upmixing, but there's something going on with the HDMI signal where it's not allowing it to be upmixed on the receiver's end. However, the receiver is at least smart enough to know when the signal is truly digital and change to whatever standard is the case (when I put in a DVD / Blu-ray), and then to switch back to speaker fill mode (5.1) for everything else that's LPCM from windows.

There's a reason I'm not doing a full-blown HTPC to do everything and this is one of them.

According to what you have posted, I would have to say it is one of those AFD-Mult mode, either Pro Logic II Movie or Music should do it. On my Pioneer VSX-1018AH, I have to switch to Standard Surround (instead of Auto), and then I can flip through things. I have Movie, Music, and Game DPL-II to choose from.

I can think of 2 things: either your receiver does not do a "best guess" upmixing without a properly encoded stereo signal (technically speaking a "Dolby Pro Logic" needs specially coded stereo data to give enough clues for the receiver to work with), or you are not using proper material to demonstrate this. On my receiver, the DPL-II algorithm is very smart in putting human dialogue to my centre speaker just like a real 5.1 stream. So depending on the scenes in the video/audio that you are playing, sometimes the surround speakers won't get a lot of audio signal fed to it, regardless of upmixing or not. Because my signals are not always properly DPL-encoded, the best guess is going to make the surrounds fairly weak.