Most screencasts are done with minimal resources, usually just a guy and his computer. This usually means crappy sound with lots of noise. Watching a screencast the other day I realized that I could remove the annoying noise I was hearing by routing the sound through AirFoil and setting the equalizer settings to kill high-frequencies.
In version 3 of AirFoil you can not only send sound to an Airport Express but also other computers, and of course, your own. To clean up any sound, just set the application to route all sound (look for “System Audio” at the bottom of the list of sources) to your own computer (by clicking on the speaker button next to the computer in the list, in most cases there will only be one, yours). You can also choose just the web browser you are watching the screencast in as the source. When you have started routing the sound open the equalizer and modify to your heart’s content.
It would be great if AirFoil had noise-cancellation built-in because you can’t always remove the noise with the equalizer, it’s a blunt instrument. I’m sure that someone more clever could come up with a way to route the sound through noise reduction filters before using a similar method.
The downside is that you get some delay in the audio, but it doesn’t seem to be the full 2.5 seconds as when you send to an Airport Express. I’ve not even noticed it most of the time when I’ve been watching programming screencasts, probably because you don’t see the speakers lips moving and that the “action” is quite slow.