I'm using the Audiophile version, and the final audio quality generated by Band-in-a-Box is still just as poor as with the WMA files. The cause is surely not in the source files, but in the post-processing the software performs when creating the mix or applying time-shifting/transposition. This includes the élastique Pro V3 algorithm and the export settings.

Does anyone know the possible reasons and how to address them?
Using élastique Pro V3 for any kind of tempo or pitch stretching can generate artifacts, even when working with high-quality WAV files, especially if there are significant pitch changes. Current algorithms aren't perfect, and acoustic pianos are particularly susceptible to digital degradation. Is it a problem that the audio regeneration is too fast? Are too many transients being created when transposing?