Sounds like a proprietary companding scheme. One would expect other data besides audio in there too. I have a friend who has figured those schemes out in the past (Alesis, Yamaha and Casio drum machines), with other machines and decoded the data to regular PCM. If you could share the images (if you still have them) that would be great. It would be a very useful resource for anyone trying to repair a machine with "ROM rot" as well I think?