Musicians are notorious for hoarding old project files. If you recorded an album in 2014 using GarageBand 10.0.3, opening that file in the modern GarageBand 10.4+ can sometimes result in missing plugin warnings, changed instrument settings, or a mixer routing mess. Installing the exact version the project was created in ensures your mix sounds exactly as you left it.
Since this is an older version, you cannot easily find it on the main App Store page if you are on a newer OS. GarageBand 10.0.3 dmg
Locate the in the App Store to see if you can legally re-download it. Troubleshoot the common "Incomplete Sound Library" error. GarageBand 10 First Run Content - Google Groups Musicians are notorious for hoarding old project files
Missing sound library or incorrect audio device. Fix: changed instrument settings