You guys have it almost completely correct. But not quite.
Protosound 2.0 will not run a DCC engine. If you own a large scale locomotive from MTH, its PS 2 equipped and will not be command controllable from a DCC system. When Protosound 3 boards come out later this year, they will be DCC compatible. That will mean you will be able to control the MTH locomotive with a DCC system. You will have access to the first 28 things the locomotive can do. See above for the list. (Greg - even the old engines like a Hudson have 50 or so distinct sound or light features. Every one is controllable from the remote. Another example is newer engines have a remote coupler that is controlled by the remote)
To get Protosound 3 in any locomotive you own now means you will take the PS 2 board out and replace it. It is not going to be a software upgrade.
PS 3.0 is not polarity sensitive. I guess that is better for some people. It will make it easier to find and load engines I suppose. You will be able to have reverse loops that are run exactly like DCC does now. (instead of having to wire them like LGB analog systems)
Another almost correct isssues. You absolutely can run your Proto 2 equipped locomotive under full DCS controll at 10v. In fact you can run it at any voltage up to 24v. There is only one thing that is affected by lower input voltage, and it isn't the sound or the smoke. Its the highest speed the locomotive can run at. I have run my hudson inside for 100s of hours off a 10 dollar 13v 3 amp radio shack DC power supply. The locomotive can not go faster than about 35 scale miles per hour. Sure, you can set the remote to 150 sMPH, but the loco will chugg along at whatever top end is limited to by the voltage potential. Everything else works exactly the same as if you had 24v input. My engines have a minimum start up voltage of about 7v. LGB digital sound locos typically started up around 6v or so. I actually run BOTH at the same time, on the same track all the time. I control the LGB engine by varing the voltage using the LGB throttle. I control the MTH using DCS. (both use the same power supply, the TIU is between the throttle and the track) Its easy, and the system was actually designed to do this. (or so I was told all those years ago)
Outdoors, I do the very same thing, except I use a much more powerfull power supply.