Where would one locate the parameters that can be used for SpeakResponseConcept? I see "HalloweenLongFall" but I can't find it in any game sound TXTs.
SpeakResponseConcept uses the files in the tf2_misc_dir.vpk, specifically in the scripts/talker/ directory.
In a few files (response_rules.txt and events.txt specifically), you'll see "Concept" in quotes in various criterion lines.
The text immediately to the right of these are concepts you can get the game to say, such as "TLK_CART_MOVING_FORWARD", "TLK_ROCKET_DESTOYED", "TLK_PLAYER_SKELETON_KING_APPEAR", or "HalloweenLongFall"
Note that not all concepts apply to all classes (such as "TLK_GRAB_BALL" which is said when a Scout recovers a Sandman Ball).
The game will just ignore it if it doesn't apply (as no response rules will match it).