I think it is a great idea to give forum owners the option to either include audio (e.g. mp3) uploads as HTML hyperlinks as done today, or just include the raw links in the post so that onebox can pick them up and make them playable.
Today forum users have to manually strip the anchor tags and paste the link in a new line by itself to make it playable by viewers. I think the default behavior should be to have it embed a player (as per onebox) and then a clickable link at the bottom of the player with the file size for downloading.
Just a suggestion, open to critique.
Great work on Discourse by the way. Amazing!