The menu you are looking for is shown at the top of the left hand column of Page 99 of the manual, but the manual is not very clear on how to get to it, so:
1. Start with the keyboard powered OFF. (This is just to ensure that we are both starting off at the same place in the menu system.)
2. Power the keyboard ON.
3. Momentarily press the FUNCTION button (by itself) one time to bring up Page 1 of the (- Function -) menu.
4. Press the RIGHT ARROW button once to go to Page 2 of the (- Function -) menu.
5. The top entry on this page (Volume) should be highlighted with a large blue dot to the left of it. If not, use the UP/DOWN ARROW buttons to select it with the large blue dot.
6. Press the ENTER button to go to Page 1 of the (- Volume -) menu.
7. The top entry on this page (Accomp Vol) should be highlighted with a large blue dot to the left of it. If not, use the UP/DOWN ARROW buttons to select it with the large blue dot.
8. Use the DATA WHEEL or +/- buttons to set the Accompaniment Volume.
9. Press the EXIT button twice to return to the main screen.