First make sure you are in Multi Play mode, the LED on the multi button is lit green when in this mode. Now press the [Multi] button again and the LED will light red indicating that you are now in mixing edit mode. Now select the desired part using the [part/element] buttons, you can then use the page buttons to select the various pages of this section. One of the pages is the Voice page. Go to the voice page and you will see a list of 8 parts, here you can make selections for each part by selecting the part and rotating the dial.