Logic X: After Monterey upgrade, only one sound plays regardless of track

After upgrading to Monterey, if I open any existing Logic X project, the only sound/instrument that plays in any track is whatever is loaded into the first track (even if I add a brand new track and a different preset, it still plays whatever is in the top slot). Any ideas??!

I have updated Logic X and tried other troubleshooting as recommended. A brand new project has no problems; it's only when I load an existing project.