If I may pop in with a couple Tips [suggestions/workarounds]...
I have run into this as well. I recently purchased PD12
[woohoo! ] and when I tried to import 8-channel audio into PD12 and it seems to only create 1-2 Audio Tracks for it (Left/Right, and possibly mixing in Center/Surround or an External). I cannot see directly if it is multiplexing channels, but it sounds alright - as long as you only want the 'main' audio tracks of a recorded game - it seems that any other audio tracks that are in a video file with multitrack audio (eg. 7.1, commentary, music, external, or multi-channel) are being ignored/discarded.
Looking at the Version Comparison for PD12 editions
here, it seems that even the Deluxe version can import 7.1 Audio, but it has to be in WAV format/tracks...
So, my suggestion is to get Mirillis to output the audio as separate WAV files, if it can do so. I know that Bandicam can do this and so can Dxtory (up to 8 tracks as separate WAV files). Then, you should be able to simply drag the audio files created in as separate tracks.
Another thing you can do is, use the software that came with your soundcard/headphones to balance your volume levels out, so that you won't need to edit the audio levels in your Project in PD12. If your software that came with your soundcard/headphones doesn't have such adjustments (or didn't come with any software at all), you can still try it with the built-in Windows Sound (eg. Recording Tab) volume levels. You can can also assign a couple of different keys for Push-To-Talk and Push-To-Record settings, between these utilities/interfaces and the programs you are using (Audio Software, Steam, Skype, etc). Overall, you should be able to get a balance you want and be able to control when it is recording from what, with different sets of Push-keys configured... All of this will just take a bit of work to set up.
Hopefully one or both of these options will work for you for now, until they patch in the ability to import multitrack-audio files (or put it in PD13).
Good luck with it!
Edit:
Added another option and clarified a bit
This message was edited 3 times. Last update was at Aug 17. 2014 16:53