Original US Carrier and Firefighter Emergency Radio Records ~30min each in .ogg for direct implementation
How to implement:
1. Make a huge triggerzone above the area you want the background sound
2. Open the Trigger menu
3. Make a Trigger on mission start
4. On the right side (what happens) Take Radio Broadcast (I don't know how exactly it is named in English, I have a German client)
5. Set the broadcast zone, set the sound file, loop on.
6. finish