Are you using any third party equipment as in a sound bar or other audio equipment or is it directly from the TV? This was posted in another thread by newapollo and seemed to help with similar issues:
Settings > Audio & Video > Dolby Audio > Follow content.
You could also set that option to Always Convert or Disabled and see if that improves the sound. If that fails try a factory reset. To factory reset go to Settings (the cog at the end of the Home menu) > System > Factory Reset.
It's actually misnamed and only a partial system reset and not a complete factory reset. You are given 2 options
Rob