This grammatical distinction is of significant consequence. A simple adjective describes a quality, but a verb phrase used as a declarative statement, or in this case, as the culmination of the preceding sentence, proclaims a truth. The lyric is not simply stating that their voices possess the quality of being unbreakable; it is a self-declared, unassailable truth about their new existence and power. This grammatical choice mirrors the film's narrative, where the song's energy has a tangible, permanent effect on the world. The lyric's structure is as final and definitive as the banishment of the demons it facilitates. It serves as the bedrock of their collective identity, a proclamation of their purpose and their unyielding resolve.