Okay. I consider myself good enough with the Hammer editor (Portal 2 authoring tools). I may be a noob, but I prefer to use ambient_generics for everything that makes noise. However, only half of the sounds (any type, e.g: GameSounds, Raw, etc.) actually have any sound in them at all; they only output silence!

Do I need to use something other than an ambient_generic to make dialogue work? Please let me know. Thanks!

Yes, you want a logic_choreographed_scene to play the dialogue. When previewing, the sounds never play inside hammer. Here's a list of all the voice lines for Portal 1 and 2, the linked file name is the same as the ingame scene usually so that helps.

Choreo scenes work better for voice lines, as they wait until other scenes are done before continuing and have OnCompletion outputs to make it easy to chain multiple lines. For GLaDOS/Announcer/ probably some of Wheatley's lines (whenever it's played over the intercom), they play at full loudness throughout the map.

Choreo scenes require an actor to "speak" the lines. I believe GLaDOS and the announcer use the same one. The global_ents instance has the required generic_actor.

Wheatley uses @sphere, the name to give him as a core, as in GLaDOS's body, or as the monitor version.

Cave Johnson requires an npc_bullseye in the map with some certain names so the audio can get quieter if you move further away from the loudspeakers. Just play the line ingame and it will output an error with the name of the bullseye you need. Put it in front of a loudspeaker model.

Thanks TeamSpen! This helped a lot. I'm not exctly sure how to use a choreographed scene, but know what to search for in the tutorials Valve posted on Steam. Thanks for your help! Radelite also gave me a few pointers, so thanks to both of you!

This is so helpful!