I recommend you use the "MP3GAIN" utility, which will let you

Anyone compared this with replaygain? They seem to be doing similar

The following text at the MP3Gain website makes me suspect that MP3Gain
implements Replay Gain for MP3 files. :-)

--8--
For a further explanation of Replay Gain (the statistical analysis done by
MP3Gain), see replaygain.hydrogenaudio.org
--8--

/Jonas