Audio Systems: AUTO REMOTE AUDIO/MICROPHONE, ambient noise level, input signals

Question
I NEED TO HAVE A 12V/24V AUDIO SYSTEM THAT WILL PLAY A CUSTOM MADE SOUNDTRACK, BUT WILL ALLOW FOR REMOTE MICROPHONE (LAPEL /TIE CLIP TYPE, INPUT SIGNALS THAT NEED TO EITHER OVERRIDE, OR BE "LOUDER THAN" THE SOUNDTRACK.
HERE IS THE SCENARIO: WE OPERATE A TOUR, WE NEED TO HAVE THE ABILITY TO PLAY BACKGROUND MUSIC/SOUNDS IN THE VEHICLE.  AT VARIOUS POINTS OF INTEREST ALONG THE ROUTE, THERE WILL BE "TOUR GUIDES" OUTSIDE OF THE VEHICLE THAT NEED TO NARRATE. THE VEHICLE CAPACITY IS AROUND 30 PERSONS, WITH A RATHER HIGH AMBIENT NOISE LEVEL.

Answer
How about something like this:

http://www.samsclub.com/shopping/navigate.do?dest=5&item=386499&pid=_Froogle&ci_...

It runs on rechargeable batteries, but you could also run it from an inverter connected to the bus or vehicle.   An inverter/converter would run around $20 and up for taking the 12v/24v up to 120 AC.

There are other portable PA devices similar to the above that you can google for.  Expect to pay around $200 and up, depending on the quality and features you get with them.

Hope this helps.
C