Press the PLUS sign as indicated near the top right corner turn up the volume
Press the MINUS sign as indicated near the center right of the touch panel to turn down the volume

Press the MUTE AUDIO button to silence the audio. Press it again to restore the audio.

Press the *BLANK SCREEN *button at the bottom right to blank the screen. Press it again to restore the picture.
Even though the class won't see anything on the screen, you will still be able to see the picture on the teaching console monitor.