Hi Grimbeorn, welcome to itsallPC
Whether you can fix this issue depends on what's causing it. Can you think of any reason why it may have started? Did you bash any of the components? Get a wire caught in anything?
Make sure the rear cable between the PC and amplifier (should be the black ended one) is in good condition. Make sure it is plugged in properly - sometimes the wires will break just before the 3.5mm jack inside if it flexes a lot or gets any punishment. Check if there is any crackle when you wiggle it.
Try plugging the front speakers into the rear speaker connectors on the amplifier (don't move the speakers). If the sound issue transfers to the front, this will rule out the speakers themselves. Then try switching the front and rear speaker cables than run from the PC to amplifier (Green and Black); if the sound issue transfers, it'll rule out the amplifier and your problem is most likely those cables.