Thanks for your help, and I'm sorry for a long post, but I'm in a pickle now, and want to provide as much info as you might need.
After playing with a few settings, I've accidentally enabled the HTPC's SPDIF out, which has apparently disabled HDMI out. Now I cannot figure out if it's a MC15 setting that will bring back HDMI, or if it's a Windows XP setting ... I believe I've looked everywhere and tried just about everything without any luck. If I configure the AVR's HDMI input to listen to the coaxial SPDIF input I have sound, but it would defeat any effort to configure the HTPC and AVR as suggested by jmone. Even while I have sound, I still have DVD playback problems ...
MC15's video parameters for DVD playback (VOB, IFO) are currently all set to "automatic", and a DVD will play ... but as soon as I ask MC to reverse or fast forward, MC will crash. If I simply restart MC again and try to play the DVD, it will play in a frame-by-frame slow motion, and only re-booting the computer will fix this. When re-booted I still have the problems with Rev & FF. DVD's were playing fine at one time, but my attempts at optimizing audio for DVD surround have messed things up and I cannot figure out how to return settings to where they were. Is there a way to reset these parameters to "as installed". I've tried removing MC15 (including registry settings), but that didn't work ...
I'm also in a quandry as to what information I can provide ... there are so many permutations of parameters, and that's for each of "Playback Method", "Audio decoders", "video decoders", "video rendoring" ... which is why they're all currently set to 'automatic' as a point to start.
So let me at least begin with glynor's and jmone's responses ...
Either MC itself or the DirectShow filters you are using for playback (probably FFDSHOW) is set to decode the multichannel sources rather than pass them through.
To begin with, I probably need to find a way to re-enable my HDMI, but I can also try to return all 3 (MC, DS, FFD) to their defaults (which is pass thru??). Regarding FDDSow, I have it installed via the CCCP (2009-09-09) package, so simply disabling the 'Dolby decoder' is different what is shown at MC's help wiki. I've never played with the CCCP settings, but they are everything is checked, except 'MT' (H.264/AVC), the SPDIF options, and "FFDShow Audio Settings" are set for '2/0/0 Stereo' with DRC disabled.
In MC, under Tools -> Options -> Video, make sure that Connection Type is set to Digital (Surround, supports AC3 and DTS).
... check!
You may also have to set the Playback Device to Default Windows rather than Same as Audio (otherwise the DSP settings you have, like JRSS 2.0, will apply to the output).
... set to "Default Windows playback device" ...
In FFDSHOW's Audio Decoder Config, make sure the Dolby Decoder is NOT enabled, and make sure the Pass-through options on the Output section are all enabled (though don't enable TrueHD and DTS HD if your AVR doesn't support those formats).
It is the CCCP UI, so I may need help specifically with the options it presents. Or, you can feel free to tell me to uninstall CCCP and suggest a different installation of FDDShow consistent with what the MC Wiki believes is installed. I also have other codecs intalled (that show under installed programs) ... "Haali Media Splitter", "CoreAVC", "XP Codec Pack 2.0.7.1", "DivX", "Ogg codecs" ... and then there's iTunes (for my iPod). I could begin with un-installing everything, except what is required (suggested) for MC15 ... but I see no need to
throw the baby out with the bathwater yet ... leastwise, not without advice.
Regarding jmones suggestions, I will consider decoding within MC playback ... but I do need to find a way to re-enable HDMI first. I've even disconnected the coaxial input, configured the AVR's HDMI input for listening to its own audio, turned everything off and on, but still nothing ... WAAAAA!
Lastly, let me be frank ... I've been reluctant to use MC for DVD playback ... I've never been able to get it to work bug-free, all the parameters for setting MC up make my head hurt, and dedicated players (e.g., WinDVD) have worked
out of the gate. Still, I'd really like to make MC & DVDs work, so I appreciate all your help. If I were to make a suggestion to JRiver, it would be to (1) create a "system info" applet that would encompass and categorize all settings, rather than what the current 'Help=>System Info' provides ... (2) create a way of saving a configuration (e.g., installation defaults) so that they can be returned to. There's also much that I don't understand, so I hope you can be patient ... all your responses are helpful and are appreciated, and I can also hope my problems will help others!!
TIA