CS6 says my clip has no audio, but it does.

Hi all,

We recently upgraded to CS6 from 5.5. I opened a project created in 5.5 and CS6 said one of the files was offline. When I tried to link it, it told me the clip had no audio - except it does, and it was created in exactly the same way as all the other clips which it is reading fine, and which are all in the same folder. No matter what I try, even deleting the file and reimporting a different version, CS6 refuses to see the audio track. Anybody know how I can fix this?

Thanks!