Acer Aspire Speaker Sound Problem

I have Acer laptop model Acer Aspire 4750G-2332G50L. From last one week I have some problem with its built-in speaker sound.When I use head-phones they work fine but as soon as I plug them out sound don't automatically get converted on laptop speakers which is usually the case.I have to plug headphones several times in and out to get it working.Can anyone guide me what could be the problem and how it could be rectified.
