Physical difference is easy; the v3s have magentic grills and aluminium dust caps, although I think you can request black dust caps if you're a current owner and need them to match.
Audio, it varies from model to model and depending on when the v2s were built. Not major, just slight improvements. I think the major changes could be heard from the ti (v1) to the v3s, particularly on the M80s.
Disclaimer: I have not compared v2 to v3 of the same model back to back, although I own v2 M80s and v3 M22 onwalls.
