You can't, I don't believe, at least directly. The one thing that will work is to host it on a server, of course, and post the URL.
A second option - and I'm not sure it will work as I've never tried it - is to save the file to your hard drive, then in a new message try and make it an attachment, just as you would a picture. I'm not sure the board software supports audio or video files, though.