All the other suggestions plus I have some thoughts too. You're going to find it rather difficult to stop the noise. As one poster suggested, solid studs would provide a bypass route for the noise, so it is best to have the ( massive as possible ) plasterboard partition supported by studs which have no direct contact with the wall. It would be even better to use resilient mounting for the stud where they do attach ( to the ceiling and floor presumably ). Gaps are to be avoided, so a means of making the gap between the partition and the surfaces it terminates at as small as possible without actually being solidly mounted to the surfaces ( another bypass route ): a labyrinth seal perhaps, with 3 laminated sheets of 10 mm plasterboard forming a U seal, as it were, on the edges, to a resilient seal attached to the walls/ceiling/floor. All very complicated, and anyway noise can still bounce off other surfaces in the room and go via the other walls. My mother is slightly deaf and I gave her a teletext TV so she can turn the subtitles on ( page 888 ). She likes the text, it isn't such a chore reading it as you might think.
Anyway, one more sugestion, to save messing around, bring the partition forward to envelop the TV, that is, put it in a TV cabinet, only make one yourself, or modify it. The design could reflect sound that exits the rear of the tv and send it forwards, so your parents could turn the volume down and still hear it as being the same volume. Also, if you construct the tv cabinet yourself, you could make it of quite massive materials, or of sandwich construction, with an inner and outer shell separated with rockwool or somesuch. Probably best to steer clear of wood, as it's not massive, unless you line it with 2mm lead, as someone suggested. Other ideas, shagpile carpet to absorb reflected sound before it gets to the party wall, or wallhangings etc. There's no simple answer!
Andy.