Apparently Sonar's Export settings dictate the resolution of the audio interface - NOT the Preferences - Audio settings nor the Preferences - File - Audio Data settings. Until now I didn't know this and couldn't figure out why the interface wasn't following certain project resolutions. Now that I know I can deal with it until it fails to work, like it did today.
 
I set the interface to 48K.
 
I open project1 with an Export setting of 44.1K and the interface switches to 44.1K. Close project.
 
I open project2 with an Export setting of 48K and the interface switches to 48K. Close project.
 
I open project3 with an Export setting of 96K and the interface switches to 96K. I change the Export setting to 48K, save and close it.
 
I reopen project3 with the Export setting of 48K and the interface stays at 96K. All audio settings in this project are at 48K and don't know what else I can do to reset the interface to 48K.
 
I'm trying to drag or export audio from this project but the files wind up twice as big as they should be. Is the project corrupted?

You can't just change a projects sample rate. You have to convert all the audio. The export settings have nothing to do with the projects audio settings. That is just to convert the export to what you want. If a project is set at 48/24 and you select export at 44.1/16 it will be converted and will take longer to export. The audio files within the project will still be 48/24.  
If you drag a 44.1 audio file into the  48 project Sonar, will convert it to 48. 
 
So make up your mind which sample rate you will use on a project and leave it be. 

Never had this problem on several computers and audiointerfaces over 20 years.
The export function has nothing to do with the audiointerface. Even at a computer with NO audiointerface you can export audio. Even at a computer with a cheap only-44-khz-interface you can nevertheless export 192-24 although the interface does not support this.
The rendering is done in the computer and not in the interface.
 
I think you have recorded your projects in different resolutions (44, 48, 96 ...). And the export setting suggests the resolution of your currently opened project.

I must be special then.
 
You all may be missing the point tho. I don't care about the project's settings, they're fine. It's the interface settings that are being changed, or not, by the current project's Export setting. What I described above is true because I verified it.
 
All audio in this one project are 24 bit 48K. All audio settings in it are 48K. But as soon as I open it, the Interface itself changes to 96K and any audio I export or drag out of the project is 96K. The other projects tested did not do this.
 
The issue occured after I changed the Export resolution in this project from 96K to 48K, so either the project retains the original setting forever or this project is corrupt.
 

Cactus Music
If you drag a 44.1 audio file into the  48 project Sonar, will convert it to 48. 

Only if you have Sonar set to do so.
 
Under Preferences - File - Audio Data your Import Bit Depth options per project are 16, 24, 32, 64, and Original.
 
You can also set the Record and Render depths per project to 16, 24, 32, or 64.

Did you look in the audiofolder what resolution your audiofiles physically have? (Windows Explorer: right-click on the file --> proberties --> details).
What says Sonar in the transport modul?
Which audiointerface do you use?
 
We digitaze thousends of tapes and vinyl since over 10 years with more than 10 computers and with different resolutions. We never had such a problem by editing the projects on 4 workstations. 

Zactly why I'm convinced the project is bad. All file attributes are confirmed in folder and Sonar Info. I'll create a new project and migrate the clips and test it again.

UPDATE - Not only was it BAD, but was also corrupting the interface drivers.
 
After reinstalling the drivers, deleting the bad project, and creating a new project to replace it, everything's back to normal.
 
Sure... if I had just done that to begin with I could have saved us all a bunch of grief and confusion.
 
My apologies!