My go-to technique has been to offer the offender a pair of headphones, saying something to insinuate that they must forgotten theirs or be too poor to afford them. Most of the time they say “oh I have headphones!” and then realize that they’ve outed themselves. (I stockpile the free headphones from gyms or airplanes, or get the $2 ones from AliExpress)

But on a plane they'll have already been asked by a flight attendant either by the time the plane takes off, or as soon as it stops climbing. So clearly this isn't working on the people this rule targets. In fact I think this rule is actually the ideal response from the airline and should be adopted everywhere, as anyone who is so unconcerned with the wellbeing of others as to play audio on their device without headphones shouldn't be allowed to fly, as they're obviously happy to fuck up everyone's day, and won't follow instructions.

Bluetooth headphones too?

This is actually a really good response though. Because the act of having a device blaring demonstrates contempt for everyone one around them. It's hard to act in a hateful way to someone who just offered you something for free.

Big "Kill them with kindness" energy.