In primary school, we had a tone that would go off on the PA system when the fire alarm would ring. We had horns in the permanent buildings, but the temp buildings didn’t have anything for the alarm, so they used the tone. They stopped using it my 4th grade year after integrities were put in the temps. Also, I don’t know what happened, but our first drill that year, the horns in the hall came on and when the tone came on on the PA, it sounded “stuck”, or better yet, like a chicken (as one teacher called it).
http://www.youtube.com/watch?v=dCJqJ9RicDI&feature=related <–the sound at the very beginning is the PA tone we had for the alarm.