Raw FM synth experiments from before Etrian Odyssey was even Etrian Odyssey. This 2007 prototype soundtrack by Yuzo Koshiro shows the series being built from scratch, and the early PCM dungeon demo from March 2006 already has that tense exploratory vibe the final game nailed.
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When you hear "Long, Quiet Corridor" mixing PCM and FM together, you can literally hear them figuring out the DS sound engine in real time. The pure FM battle outtakes go harder than you'd expect, all crunchy synth leads and rapid arpeggios.
"The Battlefield That Never Sleeps" especially. If you care about how classic dungeon crawlers found their sound, this is the development diary in audio form.