You've probably tried this, but click on "Home" then "Audio." If "Clear Voice" is set to "ON," then "Infinite Surround" is off by default. I turned "Clear Voice" off, so that I could turn on "Infinite Surround." That improved my sound. But my TV didn't sound like they were underwater, to begin with. OR, if your Infinite Surround is on, you could try turning on "Clear Voice," & see if that helps.
There is also an "auto" setting. I set mine to "On" so that the sound is more consistent from channel to channel.
Then there are the SOUND MODES in that same screen. You might have it set to GAME or something, which might alter the sound. It doesn't seem to change it much on mine.
If the above doesn't work, and if you don't have your TV hooked to outside audio speakers, then I'd say there's something wrong with either the way you did the connections, or something wrong with the TV.
But for the sound settings, the sound should work decently enough, no matter the settings. As long as you don't have the TV connected to outside speakers.
If it's your connections, you could do the "chat" feature with product support to figure that out. The connections can be tricky, depending on what you are connected to (cable box or whatever - mine goes from satellite box to DVD player to TV).