Under the app in the list, there should be a description of which types of notification an app can send. Look through the list for apps that say 'Alerts' or 'Sounds' and disable the 'Alert Tone' or 'Sounds' respectively.
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Under the app in the list, there should be a description of which types of notification an app can send. Look through the list for apps that say 'Alerts' or 'Sounds' and disable the 'Alert Tone' or 'Sounds' respectively.