on my laptop, caja does the mouse-hover-preview-mp3 thing beautifully. on my wife's dell inspiron 4010 laptop, not so much. the icon looks right when you hover, just no sound. i can open the same file in audacious or whatever, and it plays fine there. no other sound issues.
i should also mention, both our laptops are lubuntu 12.10 (with caja installed from maya repo).
checked alsamixer - nothing muted.
had my laptop sitting side-by-side with hers and (in synaptic) search/compared things like mpg, mp3, lame, gstreamer, etc, to install anything i might've missed on her machine. made everything identical...