windows movie maker audio

How to add audio to windows movie maker videos?
I have made a video using windows movie maker. When i try to add audio and select a track it says track is protected and will not add it. Does anyone know how to sort this problem please?
probably convert the audio using either a free version of acoustica, easy cd-da converter or summin else which will convert it to either wma or mp3
Change the Audio of Your Videos Using Windows Movie Maker