There is a how-to video somewhere on this site about brains. Like lots of things Mach, they are a bit quirky to set up, but once you get the hang of it, it's not hard.
I don't have the time right now to do a 100% step by step to tell you how (sorry, I'd like to, I just can't.) So the below might be a bit cryptic, depending on what you know (but is not so intentionally)
Attached is a very simple brain that does this:
-Watches for Tool Change light
(when tool change light is lit)
-starts a 5 second timer
-turns on output 6
-turns off output 6 after 5 seconds.
(restarts next time tool change light is lit)
Of course, this is assuming that your alarm is on output 6. To use this brain, put it in your mach3 brain folder. Go to mach pull down menus> operator > brain control > reload all brains (it should now show up). Highlight it and click Enable (DO NOT MISS THIS STEP).
I put it on output 6 for safety sake, as i am assuming that nothing important is on output 6. BE CAREFUL. You can change it to whatever output you want by opening the brain with Brain Editor. Click on the timer module of the brain and click the green upside down T button to reterminate the lobe and set it to what you like.
I know this all sounds mystifying, but it really is the easiest way to go short of a macropump.
-Nick