This line is certainly paradoxical because it seems contradictory. One person shouts but the other cannot hear it. This is also an example of situational irony. This is when the desired effect is the opposite of what one might expect. A shout is expected to be heard. When it is not, it is ironic. In this song, the speaker might actually hear the shouts/criticisms, but he/she chooses to ignore them. So the notion that he/she can't hear is metaphoric.
In the rest of the song, the speaker uses metaphor to explain the power of those criticisms and his/her resistance to them. The bullets are metaphors for the shouts/criticisms. The speaker is like titanium, which is a low-density but very strong transition metal. In other words, even though he appears vulnerable (low density), he is highly resilient. One could argue that the bullet metaphor is an example of hyperbole (an exaggeration). Criticisms are described as bullets to illustrate their destructive power.