amani has suggested some possible ways of investigating the software, but this could be a hardware issue.
Quite often, plugging in headphones will switch a mechanical switch to redirect sound from the speakers to the headphones. If this is the case with your system, then plugging in headphones should cause the sound to cut out; does this happen?
There is often a different impedance between the headphone jack output and the line-out jack used for speakers. It may seem obvious, but make sure you are plugging the headphones into the right socket; if you have a spare set floating around then can you try plugging speakers into the same socket?