Load your midi and then go to Song Creator. Select Step Edit and select track 10 on the top left hand side of the screen. Now look at the events list and some where there you will see Volume. Adjust this to your liking and then re-save the song.
Eileen, as I understand, I think he want/ask for a bulk solution!
If so, Ning, I don't think if is good idea to make this in bulk operation!
MixMaster was updated to work with Genos Voices, so you can use it, or MidiWorksYA (from midisoft.pl) was updated too, to Genos. I think this two solutions are faster. Of course, Eileens's solution is best for keyboard work
Regards,
Vali