Default alarm sound
This option is available only on some devices and allows you to choose the type of tone played for the alarms set in your Clock app.
Some devices do not have this setting, and allow you to set the alarm tone used for each of your alarms by going into the Clock app and going to the Alarms section. You'll also be able to set other options for each alarm such as the volume, whether the alarm can be "snoozed", and how many times.