I'm in this category
sadly seem to think it's acceptable provided it's not above conversational volume
...but not this one. I would never play any audio out loud on a busy train unless it's an urgent phone call (thankfully that's never happened). I have made one or two phone calls while on a train but I always try to find an emptier coach (fewer than 3 people) and I'm practically whispering. My go-to for train journeys is music in one earbud, minimum volume, and staring out the window. If I forget my earbuds (have done a few times) then it's silent staring... I wouldn't dare play music out loud even if I'm not in the quiet coach.
I do agree that Gen Z tend to be louder and more anti-social, but it extends to other things a lot more than just music. To name a few:
Feet on seats
Legs in the aisles
Bikes in vestibules (there's a bike compartment for a reason)
Sitting on tipup vestibule seats and then not moving to let people on/off the train
Riding bikes on platforms/in stations
Bearing in mind that these aren't Gen Z-exclusive, and I have seen people from any generation doing this (except much older people who tend to have calm, quiet conversations).
I had a young couple, probably about my age, come up to me at the barriers and ask to be let "frew" to get a train because they were driving and the road was closed. No, they didn't have tickets. I don't know what they were expecting... does a road closure entitle you to free train travel?
Headphone noise bleed is annoying but indeed the listener usually isn't aware of it, though if it's pointed out in a quiet coach the only correct recourse is to turn it down so it's no longer audible, and escalating to violence isn't really acceptable conduct.
Headphone/earbud bleed is one of my biggest concerns with listening to music on the train. I don't get how people can blast music loud enough to stop them from hearing anything without considering that others could hear it too. I once had someone in my class sit next to me with his earbuds sitting on the edge of his ears (they were the kind that wrapped around the back of the ear so they don't come out easily) and had his music on fully full volume so I could hear it over my own music. Not a fun time.
There is a 'character' well known in my area who does just that on his mountain bike as he rides at walking pace around the town with music blaring!
Unfortunately most places I've been to have their own local version of that "character". A bus station in King's Lynn comes to mind first as he was sitting on his bike blaring music loud enough to stop my conversation. The bus arrived 15 minutes later and he THANKFULLY was not on it.
But having said that, I don't expect staff to put themselves in a situation where they could potentially get assaulted. If we had security staff who had the right to take a much more robust approach towards this behaviour (e.g. fines/allowed to physically remove and/or detain offenders) they might take rules more seriously.
I'm thinking something along the lines of RPIs with more physical contact. Plain clothes, hanging around areas known for fare evasion (Stratford maybe), and ready to grab anyone that pushes. Immediate report for prosecution (pushing the barriers is a crime on its own and could likely be considered property damage) with no chance for a settlement. Pushing barriers is proof of intent, you can't "accidentally slip and fall" through one.