If you happen to listen, you will find that there are many things in life that should be silenced.
-Dumbledore
Working with audacity for the first time, you really start to appreciate just how much wacky stuff you can do with it. You can make you're voice sound like a chipmunk, or the devil himself (or herself, lets not impose gender roles on angelic beings); you can crank up the amplifier so you make yourself deaf, or you can make the voice over in your presentation practically inaudible. Why you would want to do any of these things? I have no idea, but you can, and that is justification enough.
Sometimes you've gotta get rid of that background noise; that big, loud fan, that kitchy pop music, the discussions of you're peers, that spectral, demonic voice. Never fear, audacity has a (decent) way of getting rid of that stuff.
If you've got background noise in something, open up audacity, hit file and open in the upper right to bring your audio file in.
What be this tiny bit of static that interrupts my ranting?
In the above example, a small bit of background noise was interrupting my carefully crafted voice over. You can tell its background noise because its small...and stupid. But Audacity will kill it for us.
Begin the purge.
Step one of getting rid of that little guy is to select it on the timeline with the main select tool (found in the upper middle of the toolbar; click and drag to select an area). Now that we've got it selected, hit the effects tab at the top, then hit the aptly named Noise Removal option.
Hit the noise removal button to get a noise profile of the sound.
Now that we've got the area of the clip selected, we need to get a noise profile of it; basically, the noise profile is the sound that audacity will look for; when it finds other sounds that match that profile, it will remove them. So when we select this little tiny bit of background noise, we are making it so that Audacity will be able to delete that and similar noises from the entire track when we tell it do.
As long as nothing is selected, Audacity will apply the noise removal to the entire track
When you hit get noise profile, the prompt for noise removal will go away and it will look like nothing happened, but Audacity is prepped and ready to execute that annoying little man. If we want Audacity to apply noise removal to the entire track, make sure no specific part of the track is selected. If we only want audacity to get rid of it in say, the first 5 seconds, all you have to do is select the first 5 seconds.
There's no going back now...(except for ctrl Z)
Once Audacity has the noise profile, we simply need to hit effects and noise removal again; hit the ok button to annihilate *cough* I mean remove that offensive little thing from your track.
The silence is so perfect...
Note that Audacity isn't perfect; it might leave a little itty bit that you don't want. If you care that much, you can just do the process again.
A friendly reminder...
But lastly, a warning; the noise removal tool is great for getting rid of that annoying background noise, but in doing so you are removing chunks of your audio. Thats fine if you want dead silence between words, but if you want to completely remove the sound of that fan thats making you're voice difficult to hear, you might end up with robotic, overly processed sounding audio that is no more audible than what you originally had. So watch out!