Many users have found that if the MBox2 is connected to the USB port closest to the headphone jack on these new iMacs, this problem may occur.
Power off the computer and disconnect all USB devices including the MBox. Next, connect the MBox2Mini to the USB port closest to the firewire connector (closer to the base of the imac). If you have any other USB devices, connect those to the USB ports closest to the MBox2 connection. AVOID using anything for that USB port next to the phones jack.