An audio decoding method and apparatus and an audio encoding method and apparatus which can efficiently process object-based audio signals are provided. The audio decoding method includes receiving a downmix signal and object-based side information, the downmix signal comprising at least two downmix channel signals; extracting gain information from the object-based side information and generating modification information for modifying the downmix channel signals on a channel-by-channel basis based on the gain information; and modifying the downmix channel signals by applying the modification information to the downmix channel signals.