Have you looked at the manuals?
A TV won't run external speakers directly. Besides, the TV-related audio should originate at, say, the cable Box, and go straight to your (presumed) AV receiver for the best possible decoding and reproduction.
Buzzing is an analog phenomenon and is NOT carried by optical cable in any case.
Buzzing is usally AC line noise leaking into an un-shielded cable somewhere or a floating ground. If the AC plug is reversible, try that. Sometimes the buzz is from an attached device. Make sure all audio cables are plugged in tightly at each end and routed away from or at right angles to any power cords and away from other sources of strong magnetic fields like TV's. I've also seen variable track lights induce noise but it's usually minor.
Try rotating the power cord in the wall outlet.
If it still buzzes with no cables or anything external attached there's an internal problem.
Disconnect the input(s) and see if it hums in the absence of an audio source. A bad audio cable shield or unwisely-routed audio cables will allow entrance of unwanted signals from external power sources, magnetic fields, even dimmer-controlled track lights. Sometimes, simply reversing the orientation of the ac power plug can eliminate humming.
Reverse the cables Left to Right to see if it stays with the cable or the input channel. Follow it back to the source, isolating in the same manner. Eventually you will find the entrance point of the hum. Frequently a cable's ground will oxidize over time and simply removing and reattaching the cable with a twisting motion will re-establish the shield.