nero can't normalize mp3s. nero converts mp3-files to wav before normalizing. after normalizing nero has to re-encode the normalized wav-files to mp3. therefore nero needs a mp3-codec, which is offered at nero's homepage. after testing the codec several times(30 mp3s??)you have to buy the codec(fraunhofer).best tool to normalize mp3s without re-encoding is mp3-gain.
mp3-fan