You have the main volume knob on the left next to the MFX knob and on the right there are three knobs labelled as Level, where left to right it is Lower, Upper 2 and Upper 1, then there is the Mike knob. The three Lower, Upper 2 and Upper 1 should control the volumes, Page 13 of the manual refers.
If you think of these three as Mixer control and the main Volume as the output then that may help.
If the volume issue you have is with the backing rhythm patter then Page 19 shows how to adjust its volume.
If this is not what you seek then a more precise write up of your problem will be necessary for me to gain a further understanding of your issue.