Ok. The problem came back even after cleaning, so I realize now that it probably *wasn't* the dust that was causing the problems. The problem occurred when the DVD player/home theatre was connected to the TV via HDMI at the same time that it was connected to the blue ray via optical cable.
As soon as I disconnected the DVD player from the TV and had it only connected to the blue ray player, the sound worked fine. Not exactly sure why this should be a problem, but it was.
I connected my Samsung Blu-Ray BD-D5500 to my DVD player/home theater HT-D350K with an optical digital line. Although the speakers work fine when I play a DVD directly off the DVD player, when I play a disc on the Blu-ray player, the external speakers cut out every few seconds. No problems with the sound coming out of the TV's speakers. Any ideas on the cause?