All video editing software tends to be a little bit light on audio effects. This is especially true for Windows Movie Maker, which tries to boil all parts of the video editing process to the bare minimum. Though the library is annoyingly thin there are still some times when you should try them out.
There are only three regular audio effects in Windows Movie Maker: Fade In, Fade Out, and Mute. Fade In and Fade Out are similar for video effects and audio effects. The Fade In should primarily be used when starting a sequence, just as Fade Out should be to close it out. This may also be useful for times in the middle when audio has been cut. If there are aspects of the associated video sound at the beginning or end of a clip. If this is the case you really should have some back up audio in the secondary track, possibly even some recorded background noise. In this situation you are going to have to cut up the secondary audio clip so that in the mute points for the primary audio have the secondary audio slightly louder. If you are going to work with this you have to be using the Windows Movie Maker Timeline view.
The other audio effect in Windows Movie Maker is mute. This is really just if you want to have the video without the associated audio. This may be useful for montages, music videos, and other abstract moments where the visuals are important out of context. You may just want to mute out a specific part, which will then require you to cut that part out of the clip specifically, keep it in its original order, and then mute it. You may then want to Fade Out on the clip before it and then Fade In on the one after. Just as with the Fades, you may want to have secondary audio at all times when using a mute.
All of the Windows Movie Maker audio effects are going to much more dramatic than those in other non-linear editing software. You cannot change the intensity of the audio effect and cannot layer several audio tracks. In this case you have to make sure that if you are going to use audio effects in your Windows Movie Maker project you have to looking for it to appear relatively obvious.