From what I've seen of the previous Retron systems, they're much better than Atgames' stuff--at least the sound channels all play in the same key. Sound emulation still isn't perfect, though; the NES noise channel is unusually loud on the Retron3. (It's really noticeable with Solstice, which uses the noise channel for metallic-sounding percussion...)
Though they did say that they've revamped the audio emulation for the Retron5, so...who knows.