Hmmm... the only thing I can think of is in the settings in Itunes.
Try this;
At the moment under your playback options you've probably got "sound check on" which means, many of the songs are already decreased in volume to suit the "quietest" track in your library.
Turn that off and try burning the CD again.
If this doesn't work... I'm not sure.
I've never really had that problem before (or it could just be that I haven't noticed...)
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