Hi Gary,
I know this is a little late to reply but I just joined.
What you have is a ground loop caused by different EMI potential between two or more relative grounds (your comuter/notebook and the mixer most likely). Sound cards tend to be 8-32 ohms where line inputes can be 47,000 ohms which obviously don't match. 60-cycle hum is the hum you're hearing as a result.
I had the same problem and found a product (DAK CS-1 for 29.95 plus 7 shipping) that is so simple it is rediculous. I go from a mini stereo jack from my Audigy into this little unit, into my Tascam SX-1 LE mixer/recorder and hear ZERO hum! I bought a second one for my Lambda Lexicon USB device that I use for mic input symultaniously into both the computer's USB and the Tascam as well. Cheap and extremely effective.
Check it out here: dak.com/reviews/2045story.cfm
Check out my mixes using this device and, of course, the MMV at infinite9ths.com
Best of luck to you!
Brad