Rockbox supports
Replay Gain for ID3 tagged content.
Replay gain provides volume normalization and clipping protection, but the media must be scanned and tagged with a tool that supports this in order for it to function.
MusicBrainz Picard and
foobar2000 are just two of many viable options.
Regarding your second question, I am not entirely sure what you mean exactly.
Rockbox supports
Plugins, as you almost certainly know already. Of those plugins, two of particular interest to you may be
VU meter and
Oscilloscope.
If you perhaps mean that you would like to see a VU meter type affair on the
While Playing Screen, this depends entirely on what themes are available and the individual author's preference and implementation. Themes are user submitted, so there's no way to govern what is available and who does what, but our theme engine is
very well documented and you are evry welcome to create a theme to suit your own needs and preference and then
submit it to our
Theme Gallery.
[Saint]