From what I understand, you want MC to mix 2 2 channel audio streams (e.g. Music and YouTube) and then feed that through the MC DSPs. But, MC is not set up to merge two input audio streams like that. Hence the problem.
The solution seems to be to merge the two audio streams (both 2 channel I presume) before they get to MC, so that MC can then split the merged stream into the 8 channels you want. The virtual mixer may be able to do that. But let me suggest another possible solution.
You can run 2 versions of MC, one for your music and the other for the sounds you want mixed with it. You can run exactly the same DSPs on both systems. Then, direct both versions of MC to the Behringer using non-exclusive WASAPI. That way each stream will be processed in the same way and then mixed. There could be significant timing issues between the two streams, but it seems like exact matching is not needed.
This is an unusual setup, but 2 versions of MC can run at the same time. As long as they can output to the same device, this should work. I tried it, without the DSPs, to a 2 channel output and it worked OK, using MC 28 for one input and MC29 for the other. I tried it with 2 2 channel audio only tracks and with a 2 channel audio and a 2 channel video. Both worked. I also tried YouTube on one and music on the other. Again, it worked.
This may not be a final solution, but it might be an option.
Here's a solution that worked for me...I may try the two different version jriver solution as well.
Solution (as proposed earlier that i look into voicemeeter - THANK YOU!):
1. Install voicemeeter as suggested earlier.
2. Set default system sound card to voicemeeter virtual VAIO input driver.
3. Point jriver output to voicemeeter virtual AUX input driver.
4. Point voicemeeter output to either standalone convolver software or to a different version of JRiver software running the convolution (I need to figure out which one impacts the CPU less)
5. Set standalone convolver or JRiver software running a different version and convolution to output to your soundcard/audio interface.
I wish there was something less clunky or convoluted but both of these scenarios worked. Do they impact sound quality, etc.? Unsure. Did I have to surmount some issues by mixing all my audio to the same sample rate, etc. Yes, but not a big deal.
Here's the next question I have:
Given that convolution radically reduces the output volume of your system, especially dependent on your measurement and what generates the convolution file, how can you recover that gain? My output volume is anemic when running any type of convolution. I can manually add gain (EQ, parametric EQ, etc. tons of options) within jriver but then the output is still relatively anemic even when jriver shows output hitting clipping or getting very close to clipping. Is there any way to recover any of this? If there's a sticky or guide out there for folks running into this please send it my way. I'll look in multiple forums and search this one for options as well.
Thanks!
John