Warren,
First, let me welcome you to the forums. Here you will find a wealth of knowledge on Home Automation, and some of the most exceptionally helpful people in the world that will be glad to help you in your quest.
Second, you have my sympathies with your grandfather. I, too, have dealt with the troubles of dementia and know how frustrating it can be.
Now on to your question.
I assume that by enabling a macro, that you will be using Active Home Pro on a PC running at the house.
I don't know that I would trust a motion detector for you application, as sometimes they can give false alarms thus becoming an annoyance. I'd hate for this system to turn into a "Little Boy Who Cried Wolf" scenario.
Do your grandparents keep the door closed at night? If so, why not try a door sensor (DS10A)? To use this, you will also need the OnAlert add-on to AHP. From there, you can set a macro to use the security sensor to active a "phantom module" (advance search will yield more on this). Then you can set the phantom module to trigger a macro to do whatever you want.
You might want to consider X10Wavplayer or PC Companion to play the audible alert or you can try BVC (the free version plays a nag to buy from time to time) to enact a "Listener", which can play an audio file or speach to text.
Links to all the above mentioned softwares can be found here at the forums under Third Party Add-Ons and Software.
You're on the right track, it just depends on how much Time, Love and Money you are willing to spend.
P.S. There are ways to do this without the PC. If the PC isn't an option, let us know and we'll all be glad to help!