Volume normalization is a part of
sound engineering which should be
applied on the recorded
material during the
post-production.
If this is done
digitally on a
computer, it can be done easily by an
automatic process that increases the volume on the entire recording so that the highest peaks are as close to 0
db as possible (but never over). When editing it
manually, for instance with an
analogue mixer, the recorded piece's volume is also increased, but with analogue mixing, the signal is pitched up so that the
average volume is at 0db.
There are also other ways of normalizing. For instance, if the recorded piece contains no peaks that are considerably at higher volume than the average (happens frequently when you are recording a
choir), the recording is pitched up (or down) so that the average volume is around -14db (digital) or 0db (analogue).