Extracted and edited from in-game file, all copyrights goes to HyperGryph.

Note: this is NOT the original files in the game, NOR released by HyperGryph officially.

download link for all soundtracks (including unreleased ones) : https://arknightsost.nbh.workers.dev/

For soundtracks that are not released on EN server of the game (released on CN server before), you can find it there:    • Arknights unreleased OST  
(WARNING: SPOILERS ALERT)