There's basically two broad categories of offenders, as this thread demonstrates:
1. The obnoxious - pissed up footy fans, pissed up hen parties, teens who think they're dead hard because there's a group of them and generic lout types.
2. Otherwise civilised people who seem to genuinely posses no self-awareness that watching an episode of The Big Bang Theory on loudspeaker might not be what everyone else in earshot wants to endure.
Category 1 have always been with us. If it's worse these days than in the past (I'm not sure whether or not it actually is) then that's down to worse policing and security. This is not something a lone passenger can do much about and with this category, not only is being asked to dial it down unlikely to be responded to favourably, you're also sticking a "please pick on me" target on your back and raising the risk of being assaulted.
It's actually the second category that's a more recent development and actually more concerning. I, for the life of me, can't understand how someone can genuinely not comprehend that playing music or a TV show on loud speaker will not bother anyone else. I cringe at the thought of doing it myself. But it seems to be the case. And we're not just talking about kids and teenagers here, who've grown up with the technology. It worries me that we're shifting into a "I'll do what I want and if I tread on your toes doing it, nothing personal but that's your problem not mine" culture. This is the category where it needs nipping in the bud and we need to try and make sure such behaviour is not normalised. People in this category are more likely to respond constructively to requests to turn it down and if it happens to them often, should get into the habit of doing so without being prompted. Transportation companies need to do a bit more nudging also.
The local buses have very polite notices about using earphones but they're a bit too soft to me. "Earphones Only" needs to be insisted on, loudly and firmly and that you'll be asked once and once only if in violation, otherwise you'll be kicked off.
I really do think something proactive needs to be done, otherwise, such behaviour will get ever more entrenched in the culture.