If you want the actual sound heard use the play WAV file trigger and link it to the alarm entities, (give the alarms tag names).
If you want the entities to react to the alarm then set up a mission variable, for example alarm_sounded. Have a start up trigger set the variable to false, then when someone triggers the alarm you have a trigger set the variable to true.
Then all you need is to have a trigger with the condition alarm_sounded is true, and the conditions for however you want people to react. I.e. turn hostile or follow waypoints to a certain location.