@alidan
While I was making my comment, I was wondering if I was wrong somewhere. The "Stratos Jump", huh?
Apparently, gravity is still very much effective hundreds of thousand of miles above ground, and with less air to slow you do, your terminal speed increases way beyond what normal sky divers would experience.
So, yeah. Apparently she can break the sound barrier, but it's a bit more complicated than that (given that the "sound barrier" is on a local basis, as the level of the "barrier" changes with air density) -- and she'd slow down enormously as she approaches the ground.
So she'd never break the sound barrier, twice at that, near the flying-dungeon, but she can break it as she re-enters the planet.