Please follow the steps below for the troubleshooting to the sound problem:
1. Go to [Settings] > [Sound & notification] > [Media Volume] and check if it is not set muted and then try to raise the volume.
2. Turn up the volume in the settings of apps when playing the music or video files.
3. Play other music files with the same app to test whether the sound can come out normally.
4. Play the same music file with another app to test whether the sound can come out normally.
5. Update the firmware to the latest version: Go to [Settings] > [System update] > [Check Update].
6. Access to Safe Mode by [Long press the Power Key] > [Tap and hold Power Off] > [Click OK].
If your phone works well in the Safe Mode, sound malfunction may be caused by some other 3rd-party apps. Please reboot your phone and check again if it works well.
7. Back-up your personal data and perform the factory data reset.