Here it is.. Thanks to Shady Milkman for making it happen.
This plugin basically plays an audio file then displays a csay message afterwards.
ie. admin_message this is a test would play the message.wav ("message from administration")
and display "this is a test" in centersay after the wav completes.

Included in this zip:
1.message.wav (place in mod\sound\misc dir)
2.plugin_adminmessage.sma (for editing)
3.plugin_adminmessage.amx (place in mod\dll dir)
4.sound.inc (place in Admin\scripting\include)
5.mapname.res (change name to a map on your server and place in mod\maps dir)

Make sure that you add the dll/plugin_adminmessage.amx to your plugin.ini file and you should be ready to rock!