Audio won't render

Hi,

I'm using FCP 6 and have just imported an audio file - .m4a - into my project. It is a voice recording by a Spanish friend on her Mac - so no copyright involved. Although I can listen to it in the viewer, it doesn't play when added to the timeline and the red 'not rendered' line doesn't disappear when I try to render.

Any ideas about what I can do?

Andy