The soundbar wasn't designed to be used along with the television speakers, but as a replacement.
The reason the audio may echo is because the soundbar is receiving the signal digitally, whereas the TV has to convert it to audio for the speakers in the set. This small conversion can result in an echo, so the recommendation would be to mute the TV speakers and use the soundbar for your audio.
I've seen this problem listed many times on this board with no real solution.
I have a Samsung TV (LN32A330) and recently purchased a Samsung soundbar (HT-SB1 w/ subwoofer). If I have the TV's internal speakers on, I get an echo. If I turn them off, I get a giant "Not Available" message across my screen every time I change the volume. I currently have my soundbar connected to my TV via optical cable.
This seems to be a pretty major flaw and something that's been complained about a lot on this board. Why have they not released any kind of update for this?
Thanks for any help you guys can provide!