Inserting a Pre-Roll prior to live stream

I am brand new to streaming flash technology and could use some help.

What I am trying to accomplish is have a 15 or 30 second .flv video play in pre-roll and then go into my live show stream.

This is the code that I have for my embedded player:

<script type='text/javascript' src='http://flash01.audio-stream.com/system/misc/jwplayer/swfobject.js'></script>
<div id='mediaspace'>REMOVED</div>
<script type='text/javascript'>
var so = new SWFObject('http://REMOVED/player.swf','mpl','350','216','9');
so.addParam('allowfullscreen','true');
so.addParam('allowscriptaccess','always');
so.addParam('wmode','opaque');
so.addVariable('file','8010');
so.addVariable('streamer','rtmp://REMOVED');
so.addVariable('controlbar','true');
so.addVariable('autostart','true');
so.write('mediaspace');
</script>

I have tried and failed all day today trying to figure out how to do this and I could REALLY use some help.

Ideally I would like to have a folder on my server that hosts multiple .flv files to randomly play 1 .flv file prior to launching the live stream.

If this is not possible I am OK with playing one .flv file and then I will just change it out as needed.

Can someone help me please?

Thanks in advance!