Use the "Go Advanced" options in the reply window...then use the MP3 icon (top right) to attach file...and yeah, I think you need to meet some post minimums for various BBS options.
Probably the best way to post a link (and place a player on your post, instead of members having to download) is to create a soundcloud page. Copy the URL for the song on your page you want to share. Start a new thread in MP3 Clinic, click on 'Advanced' below text window. From there, paste your URL, highlight it, then click on the Soundcloud Icon above (It's the last one). Then the Soundcloud player will show in your post, and stream the audio directly.
You may even be able to do this without the ten post minimum. I have no way to test this without creating an alias. And you know what happens then....