[Arknights OST] sys act4fun 

soundtrack of April fool's at 2023 CN server, it's very clear they overdosed NSO.
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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) : 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)