Older project won't play - gives "Unable to Open Audio Playback Device"

Set the interface to asio or whatever you prefer.
Close sonar.
Rename Aud.ini associated with the sonar install in question
Start sonar - try to open the file that exhibited the problem.

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Question1: Where do I see the Sample Rate for an existing project?    (Audio Driver Settings only shows the default sample rate for new projects, not the actual rate of the current project).
 Question2:  If there is an issue with project sample rate, how do I change the sample rate of an existing project?
 

Q1. The current project sample rate is shown in the control bar window with the audio engine button.
Q2. You have to import the audio into a project with new sample rate. I usually drag each track from the old project to the desktop. CHange the sample rate. Drag each track back. Sonar converts each track to new sample rate.
You could prob export all the audio tracks at once and then import but I rarely have more than 2 or 3 audio tracks and find it easier to drag to desktop and back.

I have confirmed that it is a sample rate problem - the Presonus doesn't support driver-controlled sample rates.
 
So I would like to convert the project's sample rate. However, it is not clear how the "dragging" process you describe (whether through desktop or media browser) would work.  The project has a mix of MIDI and audio tracks, and the audio tracks each have multiple clips spread out through time. There is also automation on the tracks. It seems like manually dragging clips is never going to get them lined up again correctly. Am I missing a trick in the procedure?
 
Is there any way to change sample rate for a project "in place"?
 
 

I don't think the automation cares about the sample rate.
Voxengo has a free utility called R8brain.
If you used per-project audio folders where all the project audio is in one folder, you should be able to apply R8brain on all audio files at once using R8brain's batch mode.
Change the project's default sample rate to the new sample rate (just to be safe).
With the project closed, convert the audio files to the new sample rate with R8Brain.
Once the files are sample-rate-converted, Sonar should open the project with the files at the their new sample rate (without making a new project).
Theoretically, R8Brain will convert ALL the audio files, including clips. All the automation should remain untouched by the conversion.
I did this years ago and it worked but maybe someone could chime in here and confirm this.
And for sure, don't do this on the orig project files. Make a copy of the entire project including audio files and work on that.
 
If you did not use per-project audio on the old project, enable per-project audio and 'save as' a new project including audio so all the project's audio is in one folder.