The system you are referring to is the Rafiki TwinFlex system.
I used to use this system in my home but I upgraded. I also wanted to have a mains device to shut down during an alarm event.
The device you need is a Twinflex Output unit...
http://www.securitywarehouse.co.uk/catalog/index.php?main_page=product_info&cPath=38_81&products_id=1665
You will then need a mains relay to link in with the relay on this output unit. This mains relay will connect to your sound system in some way... So basically, the input unit connects to a zone, the relay on the output module links to a mains relay and then that is then connected to your sound system..
Sorry I cant provide in-depth help. Somebody else will appear shortly!
Let us know how you get on.