This could be an issue with the Current Engine setting in the Pro Tools Playback Engine. This has been resolved in some scenarios by picking a different Playback Engine:
1. Launch Pro Tools 9
2. When the splash screen appears on the screen, hold down the “n” key on your keyboard, which will eventually open the Playback Engine dialog for Pro Tools.
3. At the top of this window, you will see the ‘Current Engine’ drop-down menu.
4. Choose a different Engine (such as Pro Tools Aggregate I/O)
5. Click OK.
This should allow Pro Tools to launch.
Additional Solution on Mac OS X
If the solution above fails, you can try deleting the “com.apple.audio.AggregateDevices.plist” preference on Mac OS X. You can find this preference file in the following location:
Macintosh HD > Library > Preferences
Driver Updates (Mac and Windows)
If you are upgrading to Pro Tools 9, please download and install the latest drivers for your Avid, Digidesign or M-Audio hardware from the following page: