The beep is turned off differently depending on which type of device you are working.
iOS:
click on the grey gears icon from the main screen on the device
scroll down to ScanPower and click on it
on the ScanPower Settings page toggle SILENT MODE
Android: (instructions vary depending on which kind of Android device you are using)
open the Barcode Scanning app and access settings from within the app.
on a Nexus device - click on the 3 vertical dots in the upper, right corner
on a Samsung Galaxy device click the icon in the lower left corner
scroll down to BEEP and uncheck it
