It wasn't SCSP. I remember it now - I wanted to ditch my DirectSound based AICA and do a software mixer instead. Looking for some more information on Yamaha-based chips I found ymz280b.c in MAME sources. Multiplying every magic value by 256 to get an integer constant - somehow that never occured to me.
I've brought this up for a reason. I've peeked into this file again and I see that ADPCM decoder stores loop-entry state on first pass and restores it on each jump back. I gotta try this too - but at this very moment I'm in middle of swapping one of my HDDs and that will take some time. So feel free to experiment.
Also, I have a request of sorts. Love Hina game for DC has some sequenced music running on the title screen - but it was always very broken on Makaron. If this is one of those DSF things then perhaps someone could tell me how to extract it. AO has a bit different timer engine and interrupts processing - it would help me narrow down the source of this bug if I AO plays this tune right (or at least in a different way).