The line that illustrates a simile is "She’s gone from sounding like the smoke detector through Old MacDonald and Jingle Bells."
A simile is a figurative language or a metaphor that uses comparisons to describe an event, someone, a place, etc. It aims to make the writing more colorful and interesting. We can normally identify a simile because it uses the words "like", "as" and "as in", and others, in the sentence.
In the chosen line, the narrator compares the way "she" used to sound when playing the saxophone to a "smoke detector", we can also identify the simile for the use of the word "like".