1. Take a call, and increase the volume by pressing the volume keys on the right side of the phone.
  2. Connect a compatible wired headphones. If the headphones work, remove them to continue troubleshooting the internal speakers.
  3. Set your phone to loudspeaker mode, and test again.
  4. If the problem persists, switch your phone off and on. After this operation, you may need to re-adjust the volume.