This paper proposes phase-modification (PM) method to improve the efficiency of audio coders employing an orthogonal transform. The PM acts as a preprocessing of the transform and an amount of phase to be modified can be determined by closed-loop manner. Phase-shift (PS) and partial phase-shift (PPS) methods are also described for practical implementation of proposed PM method. Performance of the PM, PS, and PPS DCT and MDCT are better than the original transform over typical audio signals in terms of transform gain.