Here's one JavaScript player: http://scmplayer.net/
A simple solution would be to use that or something like that, pick some defaults, and just make it so that any File card that is a sound format that the player knows renders in content view with a player and a link to download. A more complex solution could offer a new Audio cardtype with multiple fields to let Wagneers and editors set options on how the player appears, whether it offers the download link, etc.
In either case, one question is whether to bother with pre-HTML5 compatibility.