Automatic volume tracking for a TV

A couple of years ago I was given a non-working 50” Philips flat-panel TV. As is often the case with these things, there was actually very little wrong with it and in this instance it was a pair of bad capacitors in the power supply and it cost about $3 to fix. With the working TV, I now had another problem: The internal speakers sounded terrible! Rummaging around I found an old JVC 35 watt/channel audio amplifier/tuner from the 1980’s and using the “audio output” on the back of the TV I connected it to the amplifier and that to a pair of high-efficiency JBL 12” 4-way speakers: It sounded pretty good - lots of volume, good highs and thundering bass with “only” 35 watts (I wouldn’t need a subwoofer!) - but the volume control had no effect. Finding another cable, I then tried the headphone jack on the side of the TV, but its volume wasn’t affected!”