Welcome to iPhoneForums, Lalaland!
Use the Volume buttons to increase the volume of the sound on your iPhone, or open Control Center (slide upwards from the bottom) and adjust the volume using the slider.
If there's still no sound, connect a headset. If you can hear sound now, remove the headset and look for debris/dust/dirt in the headset jack. Clean it as good as possible.
Also clean speaker and connector.
Hope that helps.