Windows Movie Maker only sometimes plays audio

I've been uploading prerecorded audio via the record narration button, and I know it's the right type of file because it plays and I can edit it... sometimes. But when I save it and then try to go back in and continue editing later, WMM suddenly decides not to play the audio anymore.. 

This is a problem because I really don't want to have to go back and redo everything I've already done. Any ideas or suggestions?

(And yes, I am sure I don't just have the sound turned off lol.)

Thanks a ton in advance! :)