Firstly, use the headphone out. You can control volume using tv remote itself that way. (Keep volume at max in the speaker)
I have observed that all content with 2.0/2.1 sound output sounds good with proper dialog clarity in external speakers.
But content with 5.1 audio does not have dialog clarity since it is down converted to 2.1 and centre channel is affected due to that.
Try selecting 2.1 audio instead of 5.1 from the content if you are watching, that improves. If you are still left wanting, 2.1 sound bars will have the same problem. Get a minimum of 3.1 channel sound bar in that case so that the center channel is preserved.
In short, connect to the headphone out and use volume control and use 2.0 channel sound where available. If still voice clarity is missing, get 3.1 or above soundbar.