The Analytical Engine needn’t stop at numbers. It works on symbols by rules, and symbols can stand for anything whose relations can be put into rules, like the notes of music. So it could compose music, or work in any field whose rules we can write down.
Whatever a person can compute by following rules, a simple machine can compute, reading and writing symbols on a tape. And a single machine, given a description of any other on its tape, can do whatever that machine does: one universal machine can compute anything that can be computed at all.