I've downloaded the manual from the Yamaha Manual Libarary and can't find anything to suggest that you can either auto-revoice midi files or edit song track volumes.
To revoice midi files I'd point you at Michael Bedesems excellent MidiPlayer program:
http://www.svpworld.com/util_midiplayer.htm The beauty of this program is that once set up you can convert files in bulk.
However I'd be surprised if the necessary parameters for the E403 are loaded in yet. Knowing Yamaha's long and (in)glorious history of not changing things unless they are broke I'd expect the voice list in the E403 to bear a great resemblance to a previous keyboard. The tricky question (and I can't answer this) is "which previous keyboard".
I think you can use Midiplayer to twiddle volumes but I prefer to use a sequencer program like Cakewalk.
As an aside, I find the availability of on-line Manuals very valuable (big cheers to Yamaha, Behringer, Alesis, Mackie).
Do the non-attendees of this list (especially Roland and Korg, sorry if I'm wrong) appreciate that not being able to browse detailed user information online is a disincentive for potential purchasers?