Among the many common terms with musical roots:
To be “low key” means to be restrained or mellow, a term which seems to have musical origins, since lower musical keys have lower and more muted tones. Charles Dickens was among the first to use this term.
Conversely, “keyed up” means anxious, usually in anticipation. To “key up” an instrument is to tune it to a certain key.
To “play it by ear” is to improvise in a given situation, as opposed to following known rules. This began as a reference to people who can play music without referring to printed material, or without formal training.
“To pull out all the stops,” or to give something all your attention and effort, is originally referred to the workings of a pipe organ. Each pipe has a stop which can prevent pressurized air from going into that chamber, and coordinating these stops changes the sound of the music as desired. However, when all the stops are pulled out, the instrument plays at full volume and capacity.
The expression “swan song” for a comes from a long-debunked myth that swans live silent lives until just before dying, when they a sing a singularly beautiful, melancholy song. Though even many ancient Romans knew better, this idea was used by Chaucer and Shakespeare, and the phrase remains common to describe a final performance.
“Toot (or blow) your own horn,” a term indicating praise of one’s self, has roots back to the practice of announcing the arrival of an important person with trumpets.
To “march to the beat of a different drummer” is to have different principles and attitudes than those around you, and derives from Walden by Henry David Thoreau, where he writes “If a man does not keep pace with his companions, perhaps it is because he hears a different drummer.”