It's a huge issue that has thousands of prior complaints. If you feel it's the bar or TV you could return those as defective but the complaints about conversation being drowned out or low is very common.
I take it you researched this so what I write next is the usual to try.
Set the TV to send only Stereo or PCM to the bar. Look around for any settings that sound like "Clear Voice" or similar. Test that out. I do NOT expect this to fix it as a lot of content is pretty bad when it comes to voice clarity.
I'd purchased a Vizio soundbar and connected it to my Samsung Series 45 LED TV with an optical cable. I selected the external speaker on the TV's sound menu. When I watch programs the soundbars' volume fluctuates on its own. I don't know why does it do this? Is it a compatibility issue since the TV is an older model or could it be something else? I find the volume fluctuation irritating when sometimes I cannot hear parts of a conversation. I appreciate your advice and suggestions! Regards, Steve Horvath, Chicago