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Hello. Xeneder here, one of the composers. Some people used Sega Genesis style sound-fonts or VSTs for modern DAWs, but I personally used trackers (namely DefleMask) to created authentic chiptune music. I ripped some of the instruments straight out of the game, some emulators let you do that. I did some analyzing as to what were the "signature" components of the soundtrack, just muted channels one-by-one and deconstructed it from there.

There was also a huge archive that had a ton of instruments ripped from different games, including PSIV, you will have to look hard if you want to find it though.

BTW, the PCM samples can be just ripped from the ROM itself using a tool like Audacity, just open the file as RAW audio. You might have to play with the sample rate though.