Hmmm... Will the X-10 security system send out an X10 signal if a door is opened and the system is _disarmed_? How about an audible signal? It's not clear that the OP (portside) would want to keep the system armed all the time while at home. And unless an audible signal is very loud, it might not be heard in rooms of the house distant from the security console unit. If the system is armed and a door is opened, the OP might have to manually shut off the system to silence it.
The door/window sensor is useless without the X-10 security system as its signal is not detected by the TM751 or RR501 transceiver modules. However the magnetic switch supplied with the sensor could be used to actuate a PowerFlash module. (I have no idea how much a similar magnetic switch would cost if purchased separately at retail - it might be less expensive to buy the X-10 door/window sensors when they're on a good sale and just use the switches.)
I think the OP's best bet is to use a magnetic switch and PowerFlash module for each door (or several doors which are close together)
and one or more Chime Modules distributed around the house so they can be heard anywhere in the house. An alternative to a Chime Module is a UM506 Universal Module, which can be configured for a momentary "beep-beep-beep-beep" which is appreciably louder than the Chime Module. Or possibly use a Chime Module for one door and a Universal Module for another door. so they can be distinguished.