DIY siren

I'm getting a few PA speakers for the trucks this year and I would like a simple siren to go with them. I dont want a full PA/siren system with a mic and all, just a little audio device to pipe something over the speakers. We will use the CB as the mic interface part, but is there a DIY siren I could build? I considered using a cheap MP3 plugged into a splitter (CB and MP3 hooked up at the same time) and just use that to play over a few preloaded sounds. But I would prefer to have a few sounds set to a few switches...like a real siren. I figured the project would be easy enough since its just a audio player with preset sounds to a few switches, but I am having a hard time coming up with anything.