...it turns out the ADPCM decoding algorithm(OKI/Dialogic) I was using was slightly wrong(which caused a bit of extra noise and distortion in the decoded music than what should have been there).
NEC seems to have modified the delta step calculation algorithm in a subtle manner compared to the stock OKI algorithm(whether intentionally or if it was an accident, I don't know) for the PC-FX. If anyone's interested, I can release fixed copies of the music rippers.