A method for adjusting sound playback of a portable device for constancy notwithstanding different environments outputs from the portable device detectable audio signals inaudible to user and the device receives reflected audio before the portable device is actually commanded to play an audio file. A list of volume weightings for reflected audio is calculated. Before commencing playback of the audio file, the portable device obtains reference volume weightings from a list according to the current volume setting, and calculates adjustment coefficients for different frequency bands based on weightings of the reference volume list and of the reflected audio list. The audio signals of the audio file are output after adjustment. A portable device is also disclosed.