Four short lines and a couple of strange images carry a lot here. The speaker wakes up already owing something. Morning arrives like a bill, and the speaker's response is to give up their answers, as if they have nothing left to argue or explain. The title, "Mute," fits: this is someone who has stopped speaking, or can't.
The second stanza turns inward. The heart is "silent as money," which is an odd comparison. Money doesn't make a sound until it's spent or counted, and it quietly hangs over everything, much like the debt in the first line. Then comes the "little bull in a well," a picture of something strong and restless trapped in a deep, narrow space. Judging by the imagery, the speaker feels pressure and anger or longing that has nowhere to go.
The mood is tense and cramped. Nothing is shouted, but you can sense a lot of force held down in the quiet.