The process is not as simple as amplifying the waveform to make the peak level equal to 0dB. The percieved volume of a file is related to the RMS level, and some audio (particularly for radio) has the dynamic range compressed in order to produce a higher RMS and therefore "sound louder".
Some MP3 players support Replay Gain, a measure of percieved loudness, and will automatically adjust the playback volume for you.