These projects exist because modding life-critical safety equipment is usually not the best idea. You invalidate the testing originally put into the product, and it may fail when you most need it.
Yeah, everything that touches them needs to be UL listed. Commercial alarm panels have approved relay modules that can be used to output to various unregulated devices without affecting the electronics inside the panel for this reason. They also use cellular now and have to use a special modem that is UL-listed to connect.
Then put up two fire alarms, one unmodified and one where you replace the speaker with an Arduino that does whatever you want with the signal. They only cost $10 a piece. Or I guess buy one with wifi or zigbee connectivity, but then you don't have a project anymore.