Here is a link to LibriVox's Audacity tutorial - lots of good information.  There is also a section on how to clean-up background noise.  There are also other techniques for improving audio quality in this article:

• Newsforge article- Mastering Podcasts with Audacity By: Johnathon Williams
  

The techniques listed the Librivox Audacity tutorial and Johnathon Williams' article are good for salvaging poor quality audio, however, it's best not to have to resort to these when submitting audio for generating Acoustic Models for Speech Recognition. 

Ken 

--- (Edited on 11/12/2006 4:25 pm [GMT-0500] by kmaclean) ---