To confirm... It doesn't matter which HDMI input is selected (either cable box or Roku), with you being unsure of the DVD input because it doesn't get used much.
This is going to sound strange but it's worth a try... leave it on some day with the volume turned all the way down, or at least very far down, and see if it does it at the same volume. Do the same thing with it muted (not turned down, but muted with the Mute button).
Brad