Where, and in what format, are the in-game assets (sounds, models, etc.) stored for League of Legends?
Solution 1:
For assets images in general, Riot has released them. https://github.com/RiotAPI/Riot-Games-API-Developer-Assets
For audio, I'm gonna guess they are all compiled, and can't really be extracted except if you can find a way to decompile the game. Which is very hard to do.