Hi Toril,
On page 88-90 of your S975 owners manual you will find info about how to set up the microphone/guitar on your keyboard.
Gary is right - check the switch that it is set for microphones, and adjust the gain knob accordingly (see page 88).
And as Royce has indicated - you can adjust your mic settings in the Mic Setting area (see page 89 - 90).
Also, on page 57, in the Balance area, you can adjust your mic up compared to the accompaniment and the right/left voices, etc.
Also, I would assume that you have already done this, but in case you haven't you can easily download the owners manual as a PDF file.
Then you can enlarge it on your computer screen so you can read it easier...
Always have fun!