The siren is a simple block that can sound an alarm when powered by a redstone current. The power must be directly powering into the siren, meaning the redstone current has to be pointing into it, on a block directly adjacent to it, or it must be receiving power from a lever, torch or repeater, similar to how pistons detect redstone currents. The sound of the Siren is the same sound as the one used for the Industrial and Howler alarms from Tekkit, but it's pitch can be changed.
The pitch of the sound can also be adjusted by right clicking or shift-clicking it with a wrench. Furthermore, the volume of the siren can be adjusted by placing more siren blocks adjacent to the main siren block.