Sound troubles... help please

I\'m doing a radio show over the internet. I bought a 56k modem with Mic and Speaker jacks in the back so I can take phone calls.
If I plug my headset into the modem itself it works fine, but obviously that won\'t work since I need to be able to hear my co-host (who is hosting through MSN voice chat) and so does the caller, and visa versa.
So I need to connect my modem to my soundcard. So I bought some splitters and audio cables from RadioShack and hooked them up like this:
Output from modem going into Line In on soundcard.
Output from soundcard going into Mic In on modem.

It works one way (I can hear the caller though the soundcard), but not the other (soundcard output is not going into the modem).
After trying a million different methods I am stumped. I also tried using the TAD from my modem to my soundcard, and that doesn\'t work.
One of the tech guys at Best Buy said it was because the output and line in signals generate more power than the mic in signal. Thats why output - line in is working but not output - mic in.
He said maybe I can raise the gain on the microphone on the modem, on the box it says you can but I have yet to find out how.

Is there any other way I can lower the power so it\'s compatible? Any way I can raise the gain on the mic in? Any other ideas I could use?

Any thoughts at all?

Thanks.

BTW my system specs are:
Sound Blaster Audigy Gamer (http://www.soundblaster.com/products/audigy/)
AOpen FM56-PX modem (http://www.aopen.com/products/modem/fm56-px.htm)