Musically inclined Mac users—from professionals who need an easy-to-use music sketchpad to amateur enthusiasts who just want to string together prerecorded loops—have been entranced by GarageBand.

Patrick Stump was livid. On a lurching tour bus rigged with a wobbly Jenga tower of recording equipment, the singer and Fall Out Boy frontman had been trying to lay down demos for the band’s second ...