Cold, suffocating tension built almost entirely out of the DS sound chip, and it works so well it's kind of unreal. 999, composed by Shinji Hosoe, nails that feeling of being trapped somewhere you absolutely should not be.
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"Extreme Extrication" has this pulsing, mechanical urgency that makes every puzzle room feel like a countdown. "Morphogenetic Sorrow" flips everything and just breaks you with this fragile, lonely melody. Then "Chill and Rigor" lives up to its name completely.
The mathematical naming convention for the tracks is such a nice touch too. If you played 999 and still think about that final twist, go back and listen to how the music was setting it up the entire time.