Enforced by whom though? The same people that aren't enforcing other bylaws presumably.
A minor thing the rail companies could do is put up posters and scrolling messages on internal screens stating the illegality of playing sounds on board.
I do think there should be greater emphasis on "house rules". The Abellio TOCs seemed to at least publicise such things, probably because displaying "Huisregel" at the entrance to basically all businesses is totally normal in the Netherlands.
Things like what if there is a dispute about whether the blind is open or
closed, what if reservations aren't displayed etc should also feature.
The question is how? If staff are reluctant to tackle fare dodgers for fear of an unscheduled trip to hospital, why would they be any more willing to confront someone playing music out loud? The risk of aggressive retaliation is still there.
If train crew genuinely fear this and aren't just taking the easy path, suitable security backup needs to be provided for them so they feel safe doing their job.
I do find it odd that Merseyrail don't enforce it, though. To me it is a LOT more annoying than feet on the seat frame which they seem obsessed about. Indeed I find it hugely more annoying than even vaping.
FWIW I recall Merseyrail want it to be possible to charge a base penalty fare (ie £50/100 with no fare on top) for minor ASB as it would make enforcement easier - I very much support this. The old "price list" (50 quid for the red handle, 200 quid for smoking) made some sense.