Check the wires that go into the siren. Mine suddenly got quiet one day too, turns out one of the wires stripped and snapped off close to the base, probably from me adjusting the rotation of the thing on its mount one too many times. Just twisted the wire back together and taped it with electrical tape, been good for the last 2 years - until the entire mount broke off the block due to vibration and rust.
But at least the siren still works (mounted in a new ghetto location)
