If it's a Mobil station, I've found that one of the buttons surrounding the screen will mute it. Originally the button had "mute" clearly printed on it, but it

If it's a Mobil station, I've found that one of the buttons surrounding the screen will mute it. Originally the button had "mute" clearly printed on it, but it seems to have been replaced with a totally blank keycap. It still works though.