I closed Sonar and opened EZ Drummer 2 in stand-alone mode.  In Settings I saw that audio was set to MME.  I changed this to ASIO and closed EZ Drummer 2.  Then I opened one of the corrupted Sonar projects and it played without problems.
 
If routing is messed up (Ins/Outs going to the wrong things) in instrument tracks, that is still a pain but can be fixed by splitting the tracks and fixing the Ins and Outs.  The problem I was having in addition to that was 1-no audio for some VIs or 2-no audio at all 3-unable to freeze/unfreeze some synths and 4-crashes.  The change from MME to ASIO in standalone seems to have fixed these.  I will continue to monitor things and let you know.
 
I don't know if this is a 100% solution, but you should take a look if you are having issues.  Just remember that you always need to close Sonar before opening a synth in standalone.

Konradh - do you have any recent overall backup for your system drive?

I keep a full Acronis backup of everything every several days for this kind of thing. I've learned I never know when something is going to go wrong and I need everything set back to just the way it was recently. A backup tool like Acronis (I know there are others) would be the solution for me if I was in a similar jam this evening. Acronis has saved the day for me lots of times.

Just wondering if you happen to have a recent whole drive backup to restore from.

Sweetwater installed Acronis but whenever I have tried to take an image, it was unclear what to do and I didn't want to wipe out my machine. I will look for documentation. Thanks.

Bummer drag that you haven't been making regular backups of your whole hard disk collection to external drive(s). You could've had a V8!, or rather, you could have just rewound to prior to the EZD2 install. Unfortunately it appears you're in a bind. I feel for you.

Maybe after you get through this foul patch you'll take heed and start regular external backups.

I've been in the same predicament lots of times, but recovering from them has been a snap.

Good luck. It sounds awful!

Thanks for the consolation and commiseration!  The deal with Acronis is that every time I start to take an image, I get to a screen with a couple of unclear choices, both of which look like they MIGHT be taking me back to a previous image—which would really be bad.
 
Sweetwater set up the PC but didn't really have any instructions for Acronis, which I understand: it's not their product.
 
I will absolutely figure this out though in case, God Forbid, I should get a virus or some other horrible problem.
 
In the meantime, Toontrack MUST resolve this or it will really hurt their company.  The fact that they are ignoring forum posts is not a good sign.  They may be heads-down working on this, but leaving people hanging with no information is NOT the way to handle it. 
 
I am still convinced it is something to do with the audio driver set-up.

Hi Konradh. I had a similar problem with EZdrummer 2 & Sonar. Everything works great but when I go to open the project at a later date I have to open it in safe mode. After trying everything under the sun I came up with a quick fix for now. After the session I simply move the midi clip to an audio track (that I name MIDI storage Track) and remove ezdrummer from my project before closing it. This means I have to re-insert ezdrummer everytime I re-open that project but it's not really that much of a hassle. Just remember to save your drumset settings within ezdrummer. It has worked just fine and does not slow down my workflow at all ; ) I hope this was helpful.