Summary (a quick, 2-3 sentence summary): Play a sound clip when the type of ERT makes itself known via transmission. These would play alongside with the usual text appearing.
Benefits (How this will benefit the server and game as a whole): More sound is more fun. It would help distinguish the different ERTs before they arrive.
Details (Description of how you think this would work, the benefits, etc): I've included some ideas below,
Xenomorph: (I think 0:13 would be a good choice)
Iron Bears: (You knew it had to be)
WY Commandoes: (Anything with the phrase "Biotic overrun" works fine but the vast majority are good)
Mercenary team: (Seemed to fit)
Dutch's Dozen: (Either "What's the matter? The CIA got you pushing too many pencils?" or "I aint got time to bleed")
PMC: (What, you run outta marines?)
Implementation (Optional, if you have an idea how to implement it): It shouldn't be that hard... should it?