Does your soundcard have a Surround Sound or "3D" feature? If so, is it turned on all the time? If yes, it is probably adding a lot of hiss. You can find out if it's on by playing a stereo soundfile that is silent in one channel. The surround will probably put some weird signal in the channel that's supposed to be silent.
For example, the old Turtle Beach Malibu had "SRS 3D" processing which adds an awful lot of hiss and gunk, not to mention ruining any panning effects I tried to create. If your card is supported by ALSA, you should be able to turn 3D off with amixer or alsamixer. If you use OSS/Linux, you can use ossmixer. If you use OSS-lite, you may be stuck.
If you have a CS-4237B -based card such as the Turtle Beach Malibu (mine), you can try dspctrl, a little utility that lets you control the SRS and the digital output. If you have another card and you're a hacker, maybe you can use dspctrl as a guide for how to work with your soundcard (assuming the technical docs are publically available, which they're often not). If you do so, consider contributing your efforts to ALSA (if applicable).
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