Looking for Device to Boost Sound from a Home-Secu

Why not just get a better speaker to replace your old one?

Try talking to your security service. They can do almost anything if you're willing to pay for it.

As Dweller said, get the service involved. Any tampering may have a negative effect on the alarm system. If it is a private system that is not monitored, then add a headphone jack at the speaker and plug it into an amplified external speaker. Possibly a light that flashes, I have seem some audio stores that have such devices attached to phones so they observe an incoming call when demoing loud music.