Sound volume bar always come down to zero

Hi,

My hp laptop has the following specification: 

Product: HP Pavilion dv4-2101TU 

No: WH951PA#ACJ

OS: Windows 7  

For many months I cannot raise the volume slider or bar (on the top of the keyboard or through the volume icon). Once raised the volume bar, it automatically and instantly comes down to zero. If I keep pressing the volume slider bar after raising the volume, it plays the audio and as soon as I release my finger, it goes down.But, after some weeks or even months the problem gets solved by itself. But, it reappears after some days. Please some one help me out.