Look for the easy solution first
I know it may sound stupid but, try to answers the stupid questions first. Questions like...
a) Do the headphones works? (try them in other device such a TV or so)
b) Do the headphones have a volume/power control on it?
c) Is this little plugin really designed to plug headphones (refer to the speakers manual)
d) Is there some kind of software controller, speaker control panel or so that can change this pulgin volume?
e) ...
If you are sure the speakers are not working right, then
if they are cover by warranty, go to the shop and ask them to change the speakers, refund your money or so.
if not, try to carefuly open them and look for detached wires on the conector, burnt pieces or so and try to fix it (some friends can good help on that).
I suggest you again to check the most silly things. Have luck!