An apparatus for processing an audio signal and method thereof, comprising receiving a downmix signal comprising at least one normal object signal, and bitstream including object information determined when the downmix signal is generated; extracting extension type identifier indicating whether the downmix signal further comprises a multi-channel object signal, from extension part of the bitstream; when the extension type identifier indicates that the downmix signal further comprise multi-channel object signal, extracting first spatial information from the bitstream; and, transmitting at least one of the first spatial information and the second spatial information; wherein the first spatial information is determined when a multi-channel source signal are downmixed into the multi-channel object signal, wherein the second information is generated using the object information and mix information, are disclosed.