The external siren contact closure is not momentary, it’s a regular/constant normally-open switch. It closes when an alarm is active and then re-opens when the panel alarm is cancelled.
I see, yes if you need to drive a speaker you would need to add a separate speaker driver that can be controlled by the switch. The Qolsys panel “siren out” is intended for a 12vDC 300mA piezo siren so another option would be to replace the speaker with a piezo siren.