It looks like the answer is Spellsinger (1983) by Alan Dean Foster.
The premise is that aliens visit Earth seeking engineers to help them in some great task.
In a world of sentient animals and humans, the hardheaded tortoise wizard Clothahump searches across the dimensions for another kind of wizard to help defeat the looming threat posed by the armies of the Plated Folk.
Unfortunately, the guy they find is a sanitary engineer (a toilet cleaner, essentially) who gets dragged into it all.
What he gets is Jonathon Thomas Meriweather, law student, part-time would-be rock guitarist and janitor, who finds that with the use of a unique instrument called a duar, he can perform magic by playing and choosing from his well-worn repertoire of rock.