It doesn't work for me. Installed as directed (with the player_mp3.swf file in my forums root) and the player doesn't show up at all in the posts.
I viewed source on the thread page and this is the code that shows, which appears to be correct when comparing to your demo - and your demo works for me so I'm not sure what the problem is.
Code:
<object type="application/x-shockwave-flash" data="player_mp3.swf?mp3=http://www.bluesrockworld.com/musicfiles/guitarsolo1.mp3" height="20" width="200">
<param name="movie" value="player_mp3.swf?mp3=http://www.bluesrockworld.com/musicfiles/guitarsolo1.mp3">
<param name="wmode" value="transparent">
</object>