Yes, the sound can be turned off or back on!
You will need to access the settings menu by pressing the bean symbol and coffee pot buttons simultaneously for 3 seconds (further info on page 10 of the user manual) then select SE03, move the lever up or down until OFF appears on the display and then move the lever back to the central position, Press OK by pushing the lever in and the selection will flash for 2 seconds whilst being saved and the process is complete.