Split the audio coming out of your "line-in" device to get 2 sets of RCA's, and then use 2 of these:
http://www.tkk.fi/Misc/Electronics/c...linemixer.html
Then essentially bypass the soundcard, and use your line-in device all the time, and the audio would be merged with your PC audio...
Or buy a couple line mixers... (

)
The only problem is muting it, I would need more details on what your line-in device is to know if this would really work for you, but its worked for me before!
hope this helps....