If by audio, well showmessage, either I would look for quotes on the list attached (There has to be a browse option) or record your own sounds into yout_name/left 4 dead/l4d/sounds/Folder_name_you_create/filenam.wav.mp3
and then reference it with sounds/Folder_name_you_create/Filename.wav.mp3
and then make a trigger_once near where you want them to say it.. although it would still be said if the character dies, unless if its possible to filter the tirgger so only one character triggers it. That would be more realistic
use a env_soundscape and trigger it with a func_trigger at the point where you want them to say whatever.
The voice sounds are controled by script files located within the master vpk cache (open pak01_dir.vpk with gcfscape) go to the scripts folder and you will see: Game_Sounds_Gambler.txt Game_sounds_Mechanic.txt Game_Sounds_Manager.txt And so on open the txt file and look for the different soundscapes example soundscape name would be Gambler_YouAreWelcomeProducer01
OK, so i'm a traitor, but i was desperate for an answer!