I think I know EXACTLY what you are looking for.... You will need to setup a virtual sound device for every PC output you want to mix in. This can be done using software or setting up your own I/O ports to reroute the bits of info. Now I'm no good with the latter so I'll let you know of what software I found that will do just that...
Virtual Audio Cable 4.10
e2eSoft VSC - Virtual Sound Card
WiNRADiO Digital Bridgeâ„¢ Virtual Sound Card
and there's a few more. Granted, I've never used any one of them at all yet... but I want to use one tonight to record my own Voicemail Greeting message from a .WAV file.
Here is my situation. I professionally recorded my voicemail greeting and I own the .WAV audio file. Unfortunately, my phone company has no way of simply uploading the file to their servers to use it.... On another note, I lose too much quality when using the setting (PC > SPEAKERS (with my message) to PHONE (Microphone) > Voicemail recording). What I will attempt tonight will be to setup one of these virtual audio device to re-output the output of the media player to the input of the virtual audio device that will be my line in for SKYPE or another VOIP solution.
I really hope this made it a bit clearer for you... and also helped you 1 year later ;-)