With a lightning hit, it could have damaged the PC's motherboard, or any other component. Also the speakers or the power supply for the speakers.
Plug a headset into the front panel headset jack and see if you get audio there. If you do, plug the headset into the (analog) Speaker out jack on the rear of the PC and if the heaset still works then the problem is in the Speakers. If there is no sound from the front or from the rear then there is a problem in the PC. Many homeowner's policies will cover lightning damage so check that out, however, even if it covers it depending on the deductable you may still be liable for whatever damage there is.
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