Three short lines carry a lot of weight here. The speaker is playing music loudly, and the first line break, after "the," makes us wait for what's being played. Then comes the twist: "not loud enough." The volume is already high, yet it doesn't do what the speaker wants.
The poem never says what the music is supposed to do, so we have to guess. Judging by the wording, the speaker may be trying to drown something out, perhaps a thought, a silence, or a feeling that won't go away. If so, the music is a shield, and the failure of the shield is the whole point. Turning it up further wouldn't help, and the speaker seems to know it.
The mood is restless and a little desperate, but quiet in an odd way, because the poem itself is so small. The dash before the last line works like a drop in the beat. After all that noise, the poem ends on a flat admission of defeat, which makes the emptiness feel even bigger.