Jeremy, no worries, it's not openly transparent unless you look at the html source of the page directly. WordPress has a nifty feature that if you add an .mp3 URL to the text field, it will automatically format it as a player (much like embedding a youtube video), in my case I used an element wrapped around a .mp3 URL which creates a slightly different player (based on the web browser implementation of the spec) with somewhat different functionality.
As for replying to multiple sites at once on Known, on a "reply" post you'll have one URL in a field by itself, then you can click on the "Reply to a site" link under the "New Status Update" box to add another box, and so on. Thus, this particular post will send three replies. One to your response about the media player, one to your question about multiple replies, and a third which I'm sending to my original post which preceded your reply, so that it contains a full chain of the replies. There's also a "Remove URL" link in case you add too many--I don't think the post will publish if you have empty reply fields.
If you look on the wiki, there's a thing called salmentions (salmon+webmention) which programatically checks all the replies to replies and syndicates one's response everywhere so that all the posts in a chain of conversation get webmentions to have a complete record of a conversation. Few sites do this yet in practice, but if you're diligent on Known, you can effect salmentions manually.