I have a couple of these and prior to buying asked the same question as you did without a response. At a guess, Frémeaux doesn't go back to the originals for these and probably uses existing sources, e.g. the Columbia/CBS LPs or the Affinity CD set for the accompaniments. I was rather dismayed to hear some digital artefacts on at least one of these tracks (probably a Chippie Hill). Otherwise the audio quality is OK without being outstanding.