Doesn't flash have a same origin policy that would prevent you from streaming mp3s from random URLS. I thought you have to host both the player and mp3 at the same domain? Or at least have a policy file on both domains that says you can access it. I could be mistaken, but this is how I remember it from the last time I checked.
Flash can load MP3s from remote domains, but it cannot read certain metadata (eg. ID3 tags, waveform/eq/spectrum data) unless granted permissions via a crossdomain.xml policy file. For hosting, the SWF should be on the same domain as the HTML document or JS-Flash communication will be restricted (or, use the cross-domain version of the SWF.)
It may be a bit to full-featured for the small thing you are trying to accomplish but you can customize it quite a bit.
You can just listen for mp3 links in the chatroom and queue it up in soundmanager.
[1] http://www.schillmania.com/projects/soundmanager2/