I don’t think it can be a BT Vision box; a Pro box maybe, or an earlier YouView box?
But as a matter of interest, what happens if you try some other HDMI device on the HDMI port with the problem; is it still there, and you get no sound on that device, or does it work fine?
(I'm just wondering if it’s a ‘wiggle the wires’ thing, or a port failure on the TV).