After playing around with it for awhile I figured it out, by pushing the Mixer button two volume controls appear on the screen. One for Voice R1 and one for Voice R2.
I was using the Yamaha keyboard as a controller for the voices in a program called Cakewalk. But when I also used the Yamaha accompaniment at the same time one of the Yamaha voices came thru with the accompaniment.
Now that I can control the Yamaha voice volume, all is good.
Irv