I think CeeJay means voice quotes, not the text strings that the CON language calls "quotes".
The first step in the process is adding new sounds to the game. These have two parts, a
define statement that associates a name with an ID number, and a
definesound statement that specifies the filename of the sound you want to play, and some other options.
As for how the sounds are played, this gets a little bit involved because you have to start digging into the procedural scripting language of the game. To start, look at "state jib_sounds" in GAME.CON. You can see many chained
ifrnd commands that randomly choose between several sounds to play. You can fairly easily change this into a randvar call and a switch/case statement to make the code much cleaner, then you can simply increase the number of sounds possible.
This is not a very good explanation, but hopefully it's enough of a skeleton that you can do your own research by looking at the CON files, the Wiki, tutorials on RTCM, and so on.