After Screaming Mimi destroyed her larynx following the death of Angar the Screamer, Helmut Zemo took her in and recruited her to the original Thunderbolts. Because her original powers no longer functioned, Fixer designed her sonic harness based of of Klaw's weaponry to channel her remaining power. As Songbird, Mimi used the harness to create sonic constructs.
When the C.S.A offered Songbird, along with MACH-2 and Moonstone federal amnesty it was conditional on giving up her life of costumed heroics. She gave up her harness, without which she was effectively powerless; it was presumably either impounded or destroyed.
When Graviton tried to take over the world, the V-Battalion supplied Songbird with a new version of her sonic harness, based off of Fixer's original design.
The sonic harness takes Songbird's screams and focuses them, allowing her to create wings to fly, or generate any number of solid sound constructs that she can manipulate by altering the pitch and intonation of her screams.The harness interfaces with the cybernetics built directly into her neck, rendering it useless for anyone else.