The way it works is that you can set up your Exstreamer or Annuncicom to look for a priority stream on a certain port. When it sees that stream, it disconnects the normal stream and substitutes the audio from the priority stream. When the priority stream goes away the unit automatically switches back to the original stream. There's even a separate volume control for the priority stream- so you can have music playing in the background at a low level and then sue the priority stream for emergency messages at a much louder level.