Two short lines, and the whole poem turns on the jump between them. First comes a plain scene: an evening of rain so loud and unruly that "mad din" is the only way to put it. Then the speaker steps back and makes a claim about everything: "time is our tune."
The link is the sound. Rain drums on without stopping, never quite settling into a melody, and the poem suggests our lives work the same way. Time is the steady beat we all move to, whether we notice or not. The word "our" pulls the reader in, so the speaker isn't alone in listening.
The mood is hard to pin down. "Mad" suggests restlessness, even unease, yet "tune" brings in something more accepting, as though the noise has become a kind of music. The title, "Monologue," fits: the rain does the talking, or perhaps the speaker is quietly talking to themselves, sitting inside on a wet evening and thinking about how time passes.