Not sure what the A/D converter is gaining you, using its headphone output jack when the TV already has one. That probably has its volume controlled by the TVs remote, I might add...
That said, many TVs’ digital outputs will not send a Dolby Digital 5.1 encoded bitstream from any source components plugged into it. So try going into the menu of your various source components and changing their digital output from 5.1 to PCM.
Plug the headphones into the TV and ditch the converter.
Regards,
Wayne A. Pflughaupt