When I increase the volume (
without playing a sound file) I hear a strange sound like when a radio
station is out of the air ( static sound).

Check the volume control and make sure you haven't got any stray inputs
enabled (such as line, microphone etc), as this can cause extra noise in
your sound. You may need to go into the properties window (accessed from the
menus) and make sure all the checkboxes are ticked so you can see the full
range of inputs on the sound card (as there is quite a few).

Could also be your soundcard just blew some component which is causing this
extra noise. See if you can try it out in another PC to make sure if the
noise moves along with the soundcard.