This information was given to us by Jim Gutterman <jim@x-gecko.com>:
I am using a Hercules GameSurround Fortissimo II with Optical Outputs, and to date it had been working fine with WinMLS. This card seems to be the best low-cost ($50) card I have found. It has excellent audio characteristics with fully digital I/O as well as 4 channels of analog output and good mic in.
http://us.hercules.com/products/product.php3?id=9
http://us.hercules.com/products/techspec.php3?id=9
http://us.hercules.com/mediaroom/reviews/index.php3?product=9