From here you can manage the sounds of the device and can increase, lower or silence the sounds. Make sure the volume is set as high as you want and that there are no silent times set on the device.
1. What phone are you using?
2. Are you saying that there's no sound at all from anywhere (i.e., from both the speaker and the earpiece), or just the main speaker?
3. Is there any sound if you plug in a headphone?