My iphone 4 dose have no sound how to fix it?

When any app nonfiction pop up it should make a nose but is dose not do any one know how to fix it need help!

If you look at the left top side of the iPhone, you should see a switch, and then switch it off, or up,

If the doesn't work, you need to go to setting>notifications and tap on the app, then turn in sound,

If that doesn't work, then you should go to the apple retail store.