You can get rid of the background noise in Windows Movie Maker by using a third-party program called Audacity. Coupled with the FFmpeg codec library, Audacity can easily import audio from any video, and allows you to edit it to your liking and save your changes. Both software packages are free downloads, and are available for Windows, Mac and Linux computers.
Reduce Background Noise
Go to audacity.sourceforge.net, click the "Download Audacity" link and click the "Audacity Installer" link on the next page. Also, click the "FFmpeg Import/Export Library" link, which takes you to a new download page. Click the "FFmpeg Installer" link to download the audio codec library setup file.
Double-click both executable files you downloaded from the Audacity website. Run each setup separately as to avoid conflicts. Install the Audacity file first, then run the FFmpeg library setup.
Double-click the "Audacity" icon on your desktop to launch the sound editor. Click the "File" menu and select "Open." Browse for the video file you wish to edit, highlight the file and click the "Open" button.
Click the magnifying glass icon to use the Zoom tool. Zoom into the audio wave on the screen and find a single area that has only background noise and no voices or sounds you wish to keep. Click the Selection tool icon directly above the Zoom tool.
Click your cursor at the beginning of the background noise area, then, still holding the mouse button, drag the cursor to the right to highlight. Release the mouse button to stop highlighting.
Go to the "Effect" menu and click "Noise Removal." On the popup dialog box, click the "Get Noise Profile" button. Highlight the whole audio selection by pressing the "Ctrl" and "A" keys simultaneously.
Go back to the "Effect" menu, then click "Noise Removal" and the "Remove" radio button. Click "OK" to remove all background noise.
Click the "File" menu and "Export." Give the file a name in the "File Name" field. Change the "Save As Type" to "MP3 Files" and click the "Save" button.
Merge Audio and Video
Launch Windows Movie Maker.
Click the "Add Videos and Photos" icon, browse for the video file and highlight it. Click the "Open" button.
Click the "Edit" tab under the "Video Tools" area, then click the "Video Volume" icon and move the slider switch to the far left side to completely reduce the video's sound volume.
Click the "Add Music" icon, browse for the saved MP3 audio file and highlight it. Click the "Open" button. Make sure that the video and audio files line up perfectly on the timeline. As long as the two files are aligned, the audio shouldn't go out of sync with the video.
Click the "File" menu, then select "Save Movie" and click on your preferred output quality. On the dialog box, give the file a name and click the "Save" button.