one simple approach:
select all in the timeline, then use keyboard short-cuts to raise / lower the volume:
Control - /= to raise lower by 1db
Control [ / ] to raise lower by 3db
one thing, though: if you have some clips that are maxed out already, (at +12db) they WONT bet any louder, so they will appear relatively quitter when you raise everything else.
an approach to this would be to drag your edit sequence into a new sequence (with matching settings) and simple raise the volume of the nested sequence.
very simple!
nick