When you say view it in windows movie maker.. do you mean while still in editing mode or after you have published it? If the quality is worse before published.. Its most likely happening when you convert it into a readable format.. You may have forgotten to set the quality oh HQ/high.. or whatever your converter has as an option.. I think .WAV is typically one of the better quality audios that Windows movie maker can read.. but Ive always just convert the audio into MP3 and it works/sounds fine...
If its happening after you publish it may be because it wasnt saved in HD/HQ.. which if you use WMM a lot i assume you already are aware of that. But if not.. just make sure its all set to the highest quality there is before publishing it.
Worst Case Scenario.. If you dont have a converter that has the option of letting you convert both video & audio at the same time in high quality.. You may have to convert them separately. (convert the audio to MP3.. save it.. and then convert just the video in HD).. and once you do that.. Just combine the two files back together in Movie Maker.