...isn't there a 3.5mm jack silencer you can buy? Searching now and I can't find it, but I could have sworn when I was in highschool (y'know, like 25 thousand years ago) you could order a bag of like 50 of them for like $10.
It was just a 3.5mm audio plug with matte black plastic cap on the end, so you'd plug it into a TV's headphone jack and some models of TVs would be permanently silenced until someone unplugged the cap, because it was sending its audio to the 'speakers' that were just plugged in. And once plugged in, it just looked like a screw cover cap - nothing protruding. But also didn't damage anything, so you couldn't get in any real trouble for doing it.
...am I just imagining shit? If anyone knows what I'm talking about and figure out the magic search terms to actually pull it up, please drop a link to a product that can be ordered.
Edit - for visibility, I posted this comment as its own thread over on Ask.