I think del.icio.us has a great solution for a simple and easy mp3-play-button. Check http://del.icio.us/popular/system:filetype:mp3
I had a brief look at the css and js files and I think it should be relatively to create plugin for WordPress.
I can imagine that it could work like this:
- you add an mp3-link to your post
- enclose the links with li-tags and specify a mp3-butt class (class="mp3-butt")
I never wrote a plugin for WordPress although I hacked a few a bit (for xhtml compliancy for example). Maybe working with a few people on this plugin would help.
Let me know if you are interested.
Oh and btw: it should be 100% xhtml compliant of course!