Vista - No audio in Windows Movie Maker - HELP!

I have made a movie using Windows Movie Maker with audio (ripped from a CD to
MP3 format sound). It was working fine until I switched on this morning and
now the audio is not playing. I can see the track in the timeline and my
speakers are working when I use other applications (e.g. WMP), also I have
checked all the sound levels within Movie Maker and they are on full. Any
ideas why the sound is not working?

To add to this, I tried to import a couple more MP3 tracks and it says that
it cannot import the track and gives the file location. It won't even let me
import tracks that had already been imported.

I am not very good with computers so if anyone can help me I would be so
grateful. Please tell me what to do as simply as possible!

Thanks a lot,
Sarah