No sound through built-in speakers

Dear all,

the sound on my MacBookPro has gone but only through the built-in speakers (it occasionally works if I play around with the jack before pulling it out, but it soon goes again); the headphones and external speakers work fine. Digital Out Headphone port is the option available in my sound settings. Red light comes out of the headphone port when no jack is inserted (obviously) and the volume icon on the bar is greyed out. Anyone knows of a solution? Thanks alot.