Hi muppetmaster, I have got the 4000T and don’t have this problem.
I have heard that some people have had sound problems in the past, they say if you go in to the Humax 4000T setting just check that the surround sound option is turned off it cures it.
And it maybe a good idea to check the TV in settings is not on auto volume as that can cause sound issues to and don’t always work as it should on some TV’s, and make sure the Humax 4000T volume is turned up full.
Hope this helps.