A method of processing an audio signal is provided. The method includes receiving at least one digital audio stream from at least one audio source, applying at least one gain value to each of the at least one digital audio stream by a gain adjustment unit, mixing the at least one digital audio stream to which the at least one gain value has been applied, calculating a representative gain value from the at least one gain value, calculating an extra valid headroom based on the representative gain value for correction of the mixed at least one digital audio stream, and post-processing the mixed at least one digital audio stream using the extra valid headroom.