How to get Cave Johnson to speak
I've had a couple of issues getting Cave's voice lines to work properly, so I'm writing the solutions down here.
Issue 1: The voice lines don't play.
After setting up the logic_choreographed_scene entities, compile your map and play it. When you get to the points that trigger your scene entities, use the dev console and find a line that looks similar to this: CSceneEntity [logic_choreographed_scene name] unable to find actor named "cave_bomb_flings_entrance"
To solve this, create an npc_bullseye entity with the name that's shown between the quotation marks. The npc_bullseye is where Cave Johnson's voice with come from, so it's best to place the entity next to a prop_static with one of the two "old_speaker" models.
Issue 2: Cave's voice doesn't echo when it's played.
I've found that this issue is solved by placing env_soundscape entities around the map. Use this video to see which soundscapes you'd like to use.