A method, medium, and apparatus hierarchically coding/decoding audio data, such as bit sliced arithmetic coding (BSAC), such that payloads of audio data and extension data can be grouped and interleaved according to priority so that some groups of the payloads are dropped, and the remainder of groups are transmitted. Therefore, extension data that is more important than a top layer of audio data, in terms of reproducing of original sounds, can be transmitted with priority.