Considering the explicit reference to AKG with the books to the left, she's likely wearing a pair of AKG K712 Pros which are excellent reference headphones. They're open-back though so they bleed music like no one's business.
You think if Miku was listening to her own music, you think someone would've thought she was just singing outright?
Sanic_Hegehog said: You think if Miku was listening to her own music, you think someone would've thought she was just singing outright?
Wouldn't be the same. For one, that'd require you to concentrate instead of being relaxed and passively listening. For another, your own voice sounds different to yourself while speaking due to the way it echoes in your head.