There is a condensor (capicator) in the sub woofer that has to be replaced, the buzzing you hear is displaced eletricity. Open it up and look gor the largest capicator thats the one at fault. Be careful though its a bad shock if you handle it bad please be well inuslated. No its not high enough to cause damages just a bit of pain.

Sorry to tell you the ciruit is fried even if the machine you have even is on just a little like a clock and is not connected to a surge protector lighting can go directly to your electronic and do what has happened to you and that is fry your circuit I suggest if warrenty still is good take back for replacement or repair/ or just buy a new one.. but for future reference use a surge protector and here is a neat article that tells you why you should use one.

http://www.howstuffworks.com/surge-protector.htm

What this should be used for is your "computer,tv, or any other expensive electronics

I would just look at 1/4/5 those seem most important.

"loud buzzing starts for a short while and slowly fades" one of the capacitor on the amplifier final section, or preamp had blown and you get a 50 or 60 HZ loud humm that slowly fades as the AMP ICs stabilizes the signal.

Try first the following:

Test a different socket, just in case the socket has lost ground after the power surge.

If that does not work call the service number listed on the owner manual and ask modalities to fix the unit, it should have one year factory warranty on it.

Reset it.
press power buttonfor 10 seconds
remove battery and check also if it has any card like memory

It seems some circuit problem, like some out put capacitors gone weak. check them from a qualified technician.

The thunderstrom has made the system device faulty.and needs to checked internally as it needs reparing.in such cases.