Why is playing music out loud on a trail treated as such a serious offence?

The intensity comes from what people go there for, and it is worth taking seriously rather than treating as snobbery.

A large part of the point of walking in wild country is the soundscape — wind, water, birds, and the particular quality of quiet you cannot get anywhere else. It is not background to the experience, for many people it is a substantial part of the experience itself.

That explains your comparison with conversation, which is the interesting half of the question. Conversation is intermittent and it stops. People fall quiet, they pass you, the sound is variable and human and it ends. Recorded music is continuous, it does not respond to the environment, and it masks everything underneath for its entire duration.

So it is not simply louder. It occupies the whole band, continuously, and it is the specific thing many people travelled a long way to get away from.

There is a practical dimension too that rarely gets mentioned in the etiquette arguments.

Wildlife. Continuous artificial noise displaces animals from the area around the trail. That affects other people's chance of seeing anything, and in sensitive seasons — nesting, calving — it has more than aesthetic consequences.

Safety. In some terrain you genuinely want to hear things: rockfall, a river you are approaching, weather coming in, another party calling, or in bear country, everything. A speaker in your ears or on your pack removes that channel.

Worth being honest that the second point cuts both ways, because in some regions deliberate noise is recommended precisely to avoid surprising large animals. But that argues for a bell or your own voice, not a continuous soundtrack that also masks what you are trying to hear.

The reasonable middle, which almost everybody accepts:

• Headphones are fine, and nobody objects to them. If the music is for you, this solves it completely. Use one ear in terrain where hearing matters.
• Volume matters more than presence. Something audible at three metres in a busy front-country area is a different thing from a speaker audible across a valley.
• Context matters. A busy popular path near a car park is a different social space from a wilderness area three days from a road, and the expectations really are different.

Where the strong reaction is genuinely earned is a loud speaker in a remote, quiet place. That is a small number of people producing most of the annoyance, which is usually the case with these things.

One framing that has changed how I think about it: on a trail, sound is a shared resource that cannot be opted out of.

I can look away from something ugly. I can walk past litter. I cannot un-hear a speaker, and neither can anybody within a few hundred metres, for as long as it takes them to get out of range.

That asymmetry — one person's choice imposed on everyone in earshot with no way to decline — is what makes it feel like more than a preference dispute, and it is why the reaction is stronger than for things that are merely unattractive.