I've changed themes a few times and the last theme, the one I like and have settled on, isn't showing a play button or progress bar for the mp3 files in full posts (this hasn't been a problem with any other theme).
The file is clearly there and you can even make it play if you poke around with all the little text based links but the nice (and convenient) play button and progress bar aren't showing up in full posts with FIREFOX. The play button and progress bar are in fact showing up in abbreviated posts.
This page shows them:
When I click on a single option, one post - then the graphics are gone:
I see a play button on both full posts and pages with abbreviated posts on GOOGLE CHROME as well as in IE. The graphics are different but I can see the play button everywhere.