Just to update: I have since finished reading through the manual, and took note of what the capabilities are, and learning what I can go back to as I learn this keyboard.
I'm surprised by some of the features in this keyboard, and anxious to learn how to use them. When reading the manual, I found it to be less than stellar. It often references "...for details see the online reference manual". Then of course, the reference manual often refers you back to the user manual.
To the point of the original question on using the Mixer: The manuals for this topic seem to be useless for this topic. The mixer info is in chapter 8 of the reference manual. It indicates that you can change the voice (which I take that to mean I can change the existing voice in the song to a different voice) on p. 66-67. I followed the instructions, and when I then hit play...no changes, although the small icon for the voice did change to the new voice. So I figured I need to save it (although I would think you could listen to it first). Finding no save option on the mixer page, I referred back to tha manual and in t I see that to save panel settings, save to a registration memory (Panel is where I was at in the reference manual for changing the voice). So I did that. Even told it to save the song. No go.
So, I obviously am off base somewhere. Maybe I'm missinterpreting something. So, the upgrade to the 670 is a great step, they went far above my expectations. On the documentation, no more comments.