Sony NW-ZX2 Manual page 169

[Check the volume level] is displayed.
An alarm (beep) and a warning [Check the volume level] are used to protect your
ears when you turn the volume up to a level that is harmful for your ears for the
first time. You can cancel the alarm and warning by tapping the [OK] button on the
alert dialog.
You can turn up the volume after canceling the alarm and warning.
After the initial warning, the alarm and warning repeat for every 20 cumulative
hours that the volume is set to a level that is harmful for your ears. When this
happens, the volume is turned down automatically.
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