I don't know of any case books that are fully on audio. My school did give us a free account to a site called (I think) audiocasefile.com which had a bunch of cases read out, all arranged by casebook. But it didn't have every book, every case, or--more importantly--the notes after the cases.
But there are loads of audio study aids that I found very helpful, especially for the long commute to school.
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