not sure about setting it up so ALL of the songs will do it automatically but if you want to do it manually for maybe one that plays too high or low do this:
-Open itunes and go to library
-right click on song
-go down to Get Info
-go to the Options tab
-and where it says volume adjustment, can choose between +/- 100%. Need it louder then go to + etc.
-then make sure you plug it in to update of course...
Can also adjust the time it starts and ends which can be useful..