i have checked the device manager for sound devices, the device and the motherboard of my computer and it is ok, there is no sound, what is the possible problem of my computer

Welcome to D-A-L.

Is this a Desktop or Laptop?

Is this a proprietary machine? (ie; Brand_Name - Gateway, Dell, Hewlett Packard, IBM, Lenovo, Compaq, eMachine, etc.) If it is, please post the make, and the exact model number. If there is a Service Tag, post this info as well.

If the machine is a custom built Desktop;

Download, install, and run Everest Home Edition on this machine. (CLEAN FreeWare) Once it's finished loading and
running, click the + sign next to;

Motherboard.
Click on Chipset, then South Bridge. Post the information exactly as it is listed.

Multimedia.
Post the information listed under PCI / PnP Audio exactly as it is listed.