Plays one sound for a specified player, another for that player's teammates, and another for the other team(s).
SmartSound(player, playersound, teamsound, otherteamsound)
Use an entry from one of the game_sounds files in Fortressforever\scripts\. All sounds used by lua should be precached.
SmartSound(player, "yourteam.flagstolen", "yourteam.flagstolen", "otherteam.flagstolen")