From page 5-65 of the manual:
• The second stage alarm may be deactivated. - To deactivate the warning chime: Go to the 'User Settings → Driver Assistance and deselect BlindSpot Collision Warning Sound' on the LCD display. - To activate the warning chime: Go to the 'User Settings → Driver Assistance and select Blind-Spot Collision Warning Sound' on the LCD display. Informatio