In Device Manager>Sound and Audio Devices>Gebneral tab check that the Volume level is at maximum. In the Advanced tab ensure that your type of speaker setup is Enabled from the menu.
Then click on the Volume icon in the Notification tray and check that all the sliders are set to maximum where they are required and that nothing has been Muted that shouldn't be.
There are also Mute controls in Windows Media Player and similar programs - check that they have not been enabled.
It's mostly a question of checking all the sound configurations.