Volume for dual chorus playing live PSR S650

[Rod]

I'm trying to set the volume on the psr s650 for more than one voice at the same time live. This keyboard is new to me. I found the reference manual online and follow the instructions on setting each voice using the function key, but as I do it, I don't hear any difference in the volume! It sounds exactly the same for the voices when I press the keys whether I set the volumes at 0 or 100. What am I doing wrong?