Jean
The easiest method I can think of is as follows:
Program Menu>Sequencer>Range Edit>Velocity Change. Select ALL tracks for all the measures of the song and use a minus value of e.g. -30. Then select your 'quiet' track and give it a plus value of e.g. +50.
It will need a bit of 'guesstimating' with the + and - values, but should do the trick for you.
John