As far as normalizing multiple songs, as already stated your ear ,or better yet a pro mastering engineers ears is still the best bet . For home mastering, there are of course several brick wall type look ahead Mastering plugins that if not abused can help bring the level up and still prevent clipping ( going over 0db ) one that IMO does this without sound too much harshness or artifact is the Waves L2, another really good one ( unfortunately only available in RTAS format ) and the one that I use most is the Massey L 2007, very smooth and transparent ,depending on settings of course. You can hear it in action on this clip In these clips the L 2007 is set to a gain of about 7 db. With the brick wall setting of - 1. 8 db
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