In this paper, we propose a method to embed side information into MP3 bitstream using a data hiding technique. Usually, side information is transmitted in ancillary data field of an MP3 packet, which is independent of audio bitstream. When a certain amount of side information is packed into an ancillary data field, bitrate for the audio stream become smaller if we keep the total bitrate constant, which causes degradation of audio quality. We propose a new idea to embed side information into LSB of quantized MDCT coefficients. We could enhance the audio quality of the embedded signal by exploiting selective LSB substitution method. As an experimental result, we obtained better audio quality by the proposed method than simply embedding the side information into the ancillary data field.