Please take 15 minutes to write some instructions and include them with the installation.
You say "files that are attached" to a post. How can you attach an MP3 to a post that your WP installation won't let you upload because it's too large? Using the From URL tab in the media uploader doesn't work -- just creates a link to the MP3 inside the post.
Using [audio] as a shortcode therefore nets no results.
Have become convinced that finding an HTML5 audio player that actually works is like unto the quest for the Holy Grail. Whole lot of nonsense screaming "run away, run away!"