Atdm:alarm sound
From The DarkMod Wiki
The entity atdm:alarm_sound uses the script object tdm_alarm. It can be used to send audio alarms to AI as well play a sound that is audible to the player at the same time.
How to use
Place one in your map, you find it under "Speaker" in DR.
Also place an atdm:start_alarm and atdm:stop_alarm (both are under "Targets") and link these to the atdm:alarm_sound entity. Then use triggers or levers targetted at the stop/start entities to turn the alert on or off.
You can also link further entities from the atdm:alarm_sound entity. For instance lights (slowly closing) doors. These will all be turned on when the alarm is turned on, and off when the alarm is disabled.