Oppo a57 mobile phone notification message has no sound, how to set it?

This is because the ringtone notification is not turned on. Take OPPO R9S mobile phone as an example. The opening method is as follows:

First, click "Settings" on the desktop of OPPO R9S mobile phone.

Second, select the "notification and status bar" option after entering.

Third, find the item "Notification Management" and click Enter.

4. After entering, find the item "Information" and click Enter.

5. Then turn on the switch of "ringtone and vibration notification" item.