I've read, I've researched, I've asked questions.... nothing.  My projects keep changing keys.  I spend a lot of time recording these songs and pieces and they're totally screwed up and I'm really, really annoyed.  It seems to be a MIDI-related issue because I don't think my projects WITHOUT MIDI are affected. 
 
Basically, what happens is that I record the guitars and sometimes bass and use SD3 as the drum track (project template, multi-outs) and then use either True Piano or Rapture as my piano or other sound.  Generally, the recording of the actual part is fine.  Once I close out of Sonar or restart the computer (see below for gear), the project has dropped about a whole step.  I know I posted something similar to this a few days ago, but I don't get it.  What's going on?

What's your sample rate set to in your project? Sometimes Windows can get flaky when switching from 44.1 to 48 when going from sounds played in, say, Windows Media Player and then back to SONAR. If you set SONAR to your system sample rate (or vice versa), does it still happen?
 

Is it the MIDI tracks or the audio in a project with MIDI tracks too? If the former, that sounds like spurious controller or sysex messages being sent on file open.

If the latter, I'm stumped.

I agree with Tim. 'about a whole step' has to be sample rate changing from 48 to 44.1 - probably due to some multimedia app changing the rate in between SONAR sessions. But there must also be some miscommunication between SONAR and the driver about what the sample rate is or SONAR should be able to correct it. A quick search did not find a Win10 driver for the C600. If the Win8 driver hasn't been certified to work under Win10, anything is possible.

agreed, almost certainly a sample rate issue. 

When did all this start? Try to figure if anything changed
If you know the approximate date
Try restoring your computer back before that date
Couldn't hurt
Checked for malware?

Thanks for all of your help, even though I seemed irate.  I'm sorry, it was late and I was quite frustrated.  The sampling rate was set to 441000 in both my interface software and Sonar.  I didn't realize that other apps could change that sampling rate.  How does one advoid this? 
 
KPerry - it's happening when I do this:  "INSERT-MIDI TRACK, then INSERT-[whatever soft synth]." 
 
I just reinstalled Sonar and a very quick playback of one of the affected projects seems to be in the right key, however..... I'm not going to celebrate just yet.... I will update later.  Thank you all again!

If it happens again (fingers crossed it doesn't, of course), try bouncing down the project to a file and see if that's at the right pitch. If that's different as well, then I'd start to look at controller issues like was mentioned above.
 
Are you running any Windows system sounds? Boot up? Errors, etc.? Try disabling those.
 
Have you got a webcam that has its own audio driver? A lot do these days, that that could be stealing the driver focus. Go into your Device Manager and see if you can disable every audio device except for your recording gear.
 
And yeah, frustrating as hell isn't it? I've had to re-insert the keys a few times on my computer keyboard from getting angry when everything has gone south after a very long session! Perhaps I need some anger management classes or something? HAHA! I don't think anyone here could hold an angry post against you - we've all been there. 

Ok, here's an update: 
 
Reinstalled Sonar completely.  Sonar, Guitar Rig 5, Dimension Pro, Rapture.  Everything.  The good news is that my projects are now playing in the correct key. 
 
The bad (and weird) news is this:  
 
I have three projects that I've been working on.  Two of the projects were in the wrong key AND the MIDI was out of tune.  Now, those two are perfectly fine, including MIDI. 
 
But, now I'm trying to add a keyboard part to the third project and the f'ing MIDI is completely out of tune with my guitar part.  So, I'm back to my problem of the other day when I had posted that my Keystation MIDI controller was not in the correct key.  I know it's not the controller because two of my other projects are perfectly fine. 
 
So, what is going on to make soft synths out of tune with everything else?  Lord Tim, I did disable Windows sounds by the way.  I don't need them anyway.  It didn't make a difference with the MIDI issue.
 
 
 
 
 

In the event viewer are there any non-note events? It is possible to have the pitch/mod wheels drift from zero (physically) which can be recorded by SONAR.