Actually, that is among a number of features I'd like to make available. The problem is that every time I find a nice mod it requires manual changes on all other themes except the default and it is just too much work. But I heard that the forum developers are going to make the next version so that such manual tweaking doesn't need to happen. At that time, I've got a bunch of mods I'm going to be implementing.
Although, you should be able to upload small mp3s as attachments in a post... hit 'additional options'
