I think what's confusing some of the folks here is the fact that Vmo and Mmo are usually independent of one another. Vmo keeps you out of harm's way structurally (either flutter or just plain air loads) and while Mmo protects from adverse Mach effects.
Vmo and Vne are essentially the same thing. One's used on jets, the other on props and helos. Why the difference? I don't know, ask the FAA. It's in IAS because flutter depends (among other things) on dynamic pressure. However, it's not where the bad stuff happens, that's called Vd (for V "dive") and the aircraft has to be demonstrated to be structurally sound out to that speed. Then a series of upset maneuvers are flown to ensure that a pilot flying at Vmo will not exceed Vd due to inattention, disorientation, wind gusts, etc. And despite what was stated earlier, Vmo cannot be legally exceeded for a maintenance test flight (unless it's specifically stated in the flight manual, which I doubt any manufacturer would do).
Mmo is much the same, but it's based on Mach, of course. The aircraft must be demonstrated to be safe out to Md and Mmo is determined with the same upset manuevers. The difference is that handling qualities can also come into play where Mach effects are concerned, so Md may or may not be a structural limit.
And finally, I'm not sure where the concept of an "airstream" speed came from, but that would refer to TAS, and doesn't really come into play unless you're using it to calculate Mach.
I hope that clears up some of the confusion.