You don’t want your alarm system to be dependent on ST anyway, very bad idea.
ST is so unreliable and cloud based you should be glad it just works as a stand alone system. I use ST as an added security and to trigger other events that my regular alarm can’t do, such as turning a strobe light on, a z-wave siren, sending push notifications etc…
Most built-in alarm speakers are not loud enough.
I have added a self contained (vandal proof and weatherproof) siren to my alarm (GE SIMON), it is unbearably loud. You should be able to do the same, your alarm is different but they all work more or less the same way, check out my wiring diagram at this link, the only difference with your alarm is that you will need to figure out what are the trigger connections on your panel and how to connect your siren if it’s not the same model as mine. Originally this was to show how to add an ST sensor to the SIMON alarm but it also shows you how to wire an external siren