Getting buzzing sound from speaker but can here sound from them.

If the radio is powered on, put a cd in, or make sure you are on a known good radio station and connect a speaker directly to one of the pairs of speaker wires on the radio. let us know if there is still just static.

It sounds like you are getting bleed through from another device...radio, cd player...etc. This is caused by improper grounding. Some where there is a wire not connected correctly or you need to ground the whole system.

http://www.retrevo.com/support/Samsung-HT-AS720-Home-Theater-Systems-manual/id/6333ci454/t/2/
Page 10 discusses speaker connection.

Disconnect everything but the speakers and evaluate the buzz. It may be coming from external hardware or loosely connected analog cables.

Sometime juts rotating the ac plug on one device eliminates the ground loop buzz.

It sounds like the socket on your computer is becoming worn or has a loose solder joint.

If you feel comfortable with a fine tipped soldering iron and stripping your computer down I would take a look there first.

Also check the power supply for the speakers to make sure it is not getting too hot.

the protector comes on to keep the amp from further damaging itself or the speakers. The buzz you heard was probably 60HZ line buzz from the main DC supply conducting full current through one of your speakers through a damaged amplifier transistor. Your stereo will need professional repair. The protect circuit basically opens the speaker relay (and sometimes shuts down the unit) to prevent this DC voltage and current from getting to your speakers until the problem is corrected.

Sounds like you are getting quad sound in the test process that checks them, yet when in windows media player or winamp (or whatever) your rear speakers aren't producing sound: Try setting your sound drivers to do something like "speaker fill" then you are set. I'm under the belief that stereo sounds aren't being matrixed into all 4 speakers.

sounds like interferance try moving the speakers around maybe away from the monitor and try turning off any thing elec around your pc to find out maybe what is causing this

maybe some filter capacitors of the amplifier has been drying out! check for the main filter capacitors and output capacitors of your amplifier. replace them if necessary with a higher capacitance but the same voltage rating!

It sound like your getting AC noise, chances are you have some dried out capacitors