Answer:
Audio compatible with Windows Movie Maker includes:
aif, .aifc, .aiff, .asf, .au, .mp2, .mp3, .mpa, .snd, .wav, .wma
Though all the audio formats (file name extensions) listed above are compatible with WMM, the most popular format is MP3. Ensure the audio you are attempting to import into the program is one of those listed above.
Then, locate and import the file. From the Imported Media pane, drag and drop the audio into the Audio/Music track in the time-line view. Click play in the Preview pane to hear results.