Vista - Sound problem acer vista home premium

Hi, I have realised that the sound replay problem I have on my vista home
premium lies within the coputer connections on the back of the tower
somewhow.

When the speaker plug is fully inserted the sound problems remain, when you
half take out the speakers plug, the sound and vocals are great.
Obviously while you are playing a track this is OK, but, when you close
media player, with the plug in the same half out position the computer states
that the cable is unplugged.

I have tested the speakers and plug on my other computer and the playback is
fine.
The problem is the same for whatever socket I plug the speakers into on the
back of the computer, not just one. What could have changed to make all of
the rear sockets behave in such a way, very odd (they have not been damaged
physically in any way).

Any ideas?