[gjmiller]

OK, the specifics for my setup: Pro Tools 9.0.6, Mbox Pro and Windows 7.

Finally found the answer in Pro Tools help.

Not Hearing Input or Output

If you are unable to get input into or hear output from Pro Tools, the following troubleshooting steps may fix the problem.
Default I/O Setup:

Open a Pro Tools session.
Click on the Setup menu.
Click on I/O.
By default, you'll be on the Output tab. Click on the Default button towards the bottom left corner.
Click on the Input tab and then the Default button again.
Click on the Bus tab and then the Default button once again.
Click OK to save these changes.
The input and output paths for your audio interface should now be set to the default settings for your audio interface.
Double-check your input and output settings in the Mix window of Pro Tools to make sure that they're now set correctly, and test again.

Create New Session With Stereo Mix I/O Settings:

Launch Pro Tools.
From the Quick Start menu, choose 'Create Blank Session'.
Click on the '>>' button next to 'Session Parameters'.
Under 'I/O Settings' choose the 'Stereo Mix' option.
Click OK.
The session will now be created with the default input/output settings for your audio interface. Do a test recording or import some audio and see if your issue persists.

If you want, you can rename the I/O paths to how you had them set before as long as everything is working OK. In the I/O Setup, just double-click the name of the path that you want to rename, type in the new path name, and then hit Enter.