A Powerflash module to send the signal with a magnetic switch that closes when the magnet is moved away from the switch
The Universal Module can receive the X10 On signal. With the sounder set to on and in continuous mode.
If the internal sounder is not loud enough. You could wire an added external sounder to the switched contacts the Universal Module also has.
Powerflash module PF284 or X10Pro PSC01. Input Mode B; Output Mode 3
Magnetic switch wired to the Input Terminals.
Universal Module UM506 or X10Pro PUM01. Operating Mode Continuous; Sounder Only or if you want to also hear the switch click Sounder & Relay.
Both the Powerflash and Universal Module set to the same House Code/Unit Code of you choice.
If you want to look at the instruction sheets for the PUM01 and PSC01 they are on the X10Pro web site. For some reason the X10 support site only has the UM506 if memory serves me.
http://www.x10pro.com/Click on the installation instructions tab on the left side of the screen.