'Neural audio synthesis' really misses the point. This is just the same technique applied to emulating real instruments, which is already a massively well solved problem, and unlikely to result in any significant improvement.

Music synthesis, on the other hand, is an unsolved hard problem. For example, producing harmonic accompaniment to a hummed melody, or rendering a given chord sequence in different musical styles, or making a drummer that can pick up where someone's beatboxing left off..any of these have the potential to reach a huge audience.

Synthesizing real instruments basically says 'we know nothing about this space and are going to ignore all the works that's already been done in it.'