image
embedding, of course - audio files and video aren't really
"images", and probably shouldn't be in there from a first-glance point
of view.
For a "first glance", some kind of preview is useful. For audio file I
can't think of anything foolproof, just a music note image,
Maybe a music note image + the length of the audio file (if there's not
something else that shows this information). If some sound bite is 20 seconds
long, I'm far more inclined to listed to it at a computer than something
which takes a hour - but either way, length is useful 'audio-preview'
information.
-- All the best,
Nick.