This is kind of a bizarre question, but I'm sure I'm not the first to consider it. Is it possible to run the output from a sound card back into the input of the onboard audio and then have the onboard put it out?
 
First, some background. I just purchased an Asus Xonar D2X (link, it's still in the mail btw), which has no front panel audio connection. I've always used the rear ports (the front ones are a bit iffy), so that's no concern for me. However, my case (Apevia X-Cruiser) has a nice analog gauge on the front which displays the audio output. It too uses the front panel audio connector to monitor sound, so naturally it should no longer display anything once the new card is in and the onboard is disabled.
 
My theory is that I could loop the line-out from the sound card back into the mobo, which would then allow the gauge to work again. I don't care if the sound quality goes down, because the headphones would still be coming directly out of the card and the gauge is hardly sensitive enough to notice the drop.
 
Question 1: Would there be potential device/driver conflicts, seeing as I'd need to leave the onboard enabled? If not, how can I make sure that the Xonar will be the original device putting out the sound?
 
Question 2: The gauge only responds to AC'97. The sound card doesn't put this out (I believe), but would I be right in assuming that the onboard would reconvert it back (because that's what it puts out)?
 
Question 3: Is there any way that I could wire this without a simple splitter (one end going to the headphones, the other going to the motherboard)? I'll have quite a few unused ports because I'm not using 5.1/7.1 surround speakers, any way that I could push a signal out of them with no loss to the headphones?
 
In short: Sound comes out from the D2X. Clean audio goes to the headphones and also goes to the onboard audio (back into the mic jack probably). The onboard then puts out a signal to the gauge via the front panel connectors. Not a clean way of doing it, but it makes sense to someone who's never done it before.

It sounds like you're trying two get two audio devices to output audio at the same time.  I've never been able to do that with Windows XP.
 
Your scheme to get audio output readings is awfully complicated.  If you really want to monitor your audio card output, you'd probably be better off hunting around for a separate VU meter to hook up directly to the Xonar card, instead of trying to gerry-rig something with your onboard audio circuit.