Parts MIXER controls are displayed by clicking on MIXER button on the window menu. MIXER can be used in any RS mode (Performance, Tone…). When used in keyboard’s Tone, only changes to current Keyboard Part will be audible. You can select parameters to display and change by selecting from the list that is located on the left.

You can reset sliders by clicking on value label. 

Note: If there is more than one Part assigned to the same midi channel than changing one channel setting may not change the sound. Also you should pay attention to UPPER and LOWER PART settings to avoid confusion of what changes will have impact on the notes being played.