Four short lines, and the whole scene is a small surprise: drums and trumpets, usually the loudest things in a band, are tucked away in a closet like sleepy children. The title, "Diminuendo," is a musical term for getting gradually quieter, and that's exactly what the poem does. It starts with the big, brassy noise of its first line and ends on the calm words "the night."
The speaker seems relaxed, even fond. Calling the closet "my closet" and describing the instruments as "settling in" makes them feel like guests or pets being put to bed. There's a gentle humor in picturing a drum kit and trumpets getting comfortable together. The mood is hushed and cozy, a day winding down. It's also possible to read the closet as a kind of mind, with the noise of the day finally dying away, but the poem never says so. It simply lets the sound fade, and the reader's own quiet follows.