Hey Edubfloyd,
Thanks for clarifying. Does this happen with all projects? You can create a new project to test this out, and see if it continues. If it doesn't happen in a new project, then it's likely that something in the project you've created has become corrupt. If it is just the project alone, then you can begin removing content from the project, and testing the playback. Before doing this, please back up your computer by using Time Machine.
Use Time Machine to back up or restore your Mac
Also, please duplicate the GarageBand project in Finder by selecting the file, then going File > Duplicate. Then open your project, and start deleting content in the project, one half at a time. If the issue persists after deleting the one half, then the issue may be in the other, so undo the deletion and try again with the other half. Continue with this process until you hopefully find the source of what is causing the issue in the project.
Now on the other hand, if the test project has the same issue, then I suggest going through the process in the link below to test again in a new user. In the new user, create a test project as you did in the original user account, and see if the issue persists.
You can figure out if unexpected behavior is related to user file or setting by trying to reproduce the issue from another user account. This process includes creating a new user account, logging in to it, and testing for the issue.
Create a test user account
- From the Apple menu, choose System Preferences.
- Click the Users & Groups icon in the System Preferences window.
- Click the lock icon, then enter an administrator name and password.
- Click the Add button (+) below the list of users.
- Choose a type of user from the New Account pop-up menu.
- Give the user a full name, account name, and password.
- Click OK.
- Close the System Preferences window.
If there are documents from your original user account that you want to test with, place a copy of these items in the Shared folder in the Users folder. Press the Option key while dragging a file to this folder to create a copy.
Log in to the test user account
Log out of your current user account by choosing Log Out from the Apple menu, then log in to the new account you created. If you're prompted to sign in with your iCloud account or Apple ID when you log in, skip this step.
Try reproducing the issue
Try the same steps that caused the unexpected behavior to appear before.
How to test an issue in another user account on your Mac
Please let us know how it goes!
Take care.