Why can't I hear anything in Logic Pro?

I just installed Logic Studio 9. I have about 10 Logic projects that I created with a previous version of Logic. I can’t get any of them to play back sound.

Troubleshooting steps I’ve done so far: My MacBook is able to play all other sound. I’ve checked the “mute” and “solo” buttons, and none of them are selected (or flashing). I’ve tried turning on and off the “S” button in the transport bar. The volume meter does display a high volume during playback. The volumes in Logic (master volume and track automation) are turned up. I’ve searched in Help and on Google and haven’t found anyone addressing this. I’ve tried playing several different projects. I’ve repeatedly restarted the application, and I restarted my MacBook.

Details in case they’re useful: I’m using Logic 9.1.1 with Mac OS 10.6.8. There are audio and MIDI tracks, as well as imported sound files (AIFF).

Any suggestions?