Alarm model number MSS5M1 has one positive terminal (rated at 5 ± 0.5 Vdc) and three ground control pins- (-) Low, Medium, & High. The (-) pin must be grounded in order to activate all three priority sounds. For example, just grounding the (-) pin will result in the low priority sound being activated. If the (-) and Medium pins are grounded, then the medium priority sound will be activated. Finally, if the (-) and the High pins are grounded, the high priority sound will be activated. If all three control pins are grounded, then the high priority sound will be activated. If you cannot ground the (-) pin and one of the other two pins simultaneously, ground the (-) pin first.
How can this control be accomplished in the medical equipment? The peak current draw of the MSS5M1 alarm will be 250 mA no matter which control pins are activated. Therefore, you can use transistors to act as the buffer between the control signal and the alarm. Just make sure that the transistors are able to hand the 250 mA of peak current.
Below is an example of how the control circuitry could be implemented: