Follow the steps in this article to (de)activate the notification sound for new online orders.

When activated, Lightspeed will play a sound until you accept the online order. To change this default setting, go to Devices ➜ Restaurant App Settings ➜ select the profile for SOUND EFFECTS AND ALERTS.
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To change the ringtone and volume of the notification sound, open the Settings app on the iPad and go to Sounds.