This is cool... Don't you hate it when you are listening to your mp3 collection and the volume levels are all different?
I've been using the tool normalize for quite a while to adjust wav file volume levels when burning cds - works great.
Well I just discovered the latest version of normalize works on mp3s too. And the way it works is cool - it computes a dB adjustment for the file, but instead of decoding and reencoding the file to apply it (which would degrade the audio), it writes the adjustment into a special standard id3v2 Relative Volume Adjustment tag.
Then any mp3 player can check that tag and apply the adjustment on playback.
I adjusted all 1200 or so mp3s on my system - took about 3 hours. Now everything is consistently the same relative volume.
Of course, the downside is that apparently the only thing that operates on the special tag right now is a plugin from the normalize author for xmms. Still, it's a standard id3 tag, so expect it in more players as id3v2 becomes more widespread.