Normalising them to 0dB has dubious advantage, and many, many disadvantages (it's the audio equivalent of making everyone the same high -exactly the height of the ceiling -see how quickly that wears thin
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You'd be better normalising each track to -18dB
RMS -it would be more meaningful prior to mixing. Better still normalise each clip by ear to roughly the level required for your idea of "the right level"
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