Yeah, unless some old NEC employee finds all the project materials like the original source recordings, the raw ADPCM binary stream from the Ys Book I & II data track is the best that we'll ever get...
That's if guessing that they were recorded in wave first; they could've went directly to ADPCM and so there might not actually exist a recording that's better in quality. But nah, that's a silly bet since some of the voice-acting is on the redbook audio tracks, so they were recording to wave (
AKA 16-bit stereo 44100 Hz PCM) first in those cases, thus original wave files likely do exist of everything on some old NEC USA computer right now, but who knows where it's all at...
Would be cool if companies thought to donate old hard drives and what not with stuff like that to videogame historical/preservation groups but obviously things like that would rate very low, if ever, on a priority list. Would be a shame if it all just sort of got thrown out with the trash. In 1990, what kind of PC would they have been using for a localization project, like say a Tandy 1000 ?? How long would you keep equipment like that around as a company ?? It's probably safe to say most stuff got nuked after they got out of the videogame business on the US side of things...