I'm guessing they meant past bookings but grammatically it sounded like MJ had an upcoming concert booked at the arena which would be...peculiar, to say the least.
I've seen Stevie Wonder and Bruce Springsteen at the Glendale arena and Rush several times at America West and my hearing is awful, but concerts in enclosed spaces make for awful and muffled acoustics. I'm also a cheapass and sit up in the rafters so maybe that's part of the problem.