How to normalize your MP3 volume levels (the right way)
The problem is that the vast majority of audio player programs use peak amplitude analysis to determine how "loud" a song is and normalize based on that, rather than doing a more comprehensive analysis of the frequencies that impact how loud the music actually sounds. But even those that do a more comprehensive analysis fail to consider songs within the context of the album they belong to.
Luckily, there's a great little utility called MP3 Gain that does lossless volume analysis and adjustment based on David Robinson's Replay Gain algorithm, as well as allowing for the volume of entire albums to be processed. MP3 Gain has been around for a number of years, and it doesn't appear that it is being actively developed anymore, but since MP3 technology hasn't changed much either that doesn't matter for our purposes.
So, how do you fix an entire library of music? Pretty easy, actually. MP3 Gain offers batch processing options that allow you to process an entire folder at once. But if you collect music albums, you'll want to do those separately. The reason is that MP3Gain offers two modes: Radio and Album. Radio analysis and gain adjustment will adjust all songs to the same maximum decibel level. Album mode analyzes all of the songs in an album, essentially considering them to be one entire song, then adjusts them all to a maximum decibel level, but maintains their relative volume.