The question is, has changing the resolution helped?
Are you sure you were originally on 1080p and not 1080i, as its 1080p which normally causes issues?
Is the TV capable of full HD?
Within the TV itself, the interlaced (i) is converted to progressive anyway, as that is how flat screen displays are driven.
The old CRT (Cathode Ray Tube) TV sets built the picture up over two frames, as that is all the scan coils were capable of. Modern flat screen TV sets do not have that limitation.
In practice, you would be hard pressed to see the difference between 1080p and 1080i, except perhaps on very fast motion, even then you would have difficulty noticing it.
Thanks for trying.
If everything else has been tried, then that just points to a possible issue with the TV, especially as it only affects the sound.
Have you tried it on a different TV, at any point?