Reason: 1. Volume too lower
2. Inserted an earphone
3. Single problem
1. Depress the volume button, check if volume is too lower.
2. Check whether pressed the mute button, try to depress it again.
3. Turn to another program to see whether it's a single problem.
4. Check if it has inserted an earphone.
1. Increase the volume
2. Press the mute button
3. Check the single wire
4. Pull the earphone out