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.

In early 2004, Steve Jobs enthusiastically unveiled GarageBand, a program that gave even musical novices the power to fashion dynamic soundtracks. But as marvelous as GarageBand was, some users were ...