I want to incorporate a noise reduction software module into my
system. The idea is, the module would pick audio data from the sound
card, filter it to reduce noise, and make the output available to all
the audio apps, e.g. sound recording or voice chat. I'm a novice to
driver writing so any advice would be hugely appreciated!

Tom

Hey! Great question!  I've run across problems like this myself. as it
turns out, the issue i had was my onboard sound card picking up some
electrical feedback from my chipset. No idea how to fix it - so i
purchased a new sound card. the solution worked quite well. However,
that doesn't answer your question, so here's my best answer:

try this. http://www.ntonyx.com/vac.htm

Good luck!

Hans

If you need to use audio out to Voice chat/as replacement to
Microphone only u need is to change the sound settings. In sound
settings  select 2 channel then select recorder as wave or Stereo mix.
High quality sound will be transfered to Chat Clients.

If the Noise is already in mp3/audio outputs Check the CD-Drive sounds
on/off. It may cause some noise. Try to disconnect CD rom audio cable.
There is no need of it