[mythtv] Audio level normalization

[Reza Naima]

I'm keen to add some code to normalize audio levels -- either before
encoding -- or during playback.  I'm sick of having to constantly adjust
the volume at night so as not to disturb others sleeping in the house
whenever an explosion goes off in some movie -- only not to be able to
hear the dialog in the next scene.  I was wondering if there was any
other interest for this feature, and peoples opinions on where in the
code this should be inserted..

Reza