Some, many? most? cell phones have a builtin FM receiver so in many cases no additional HW should be needed

[0] https://duckduckgo.com/?q=cell+phone+FM+tuner

FM audio on yoru phone usually relied on the headphone cable being used as an antenna to receive the FM signal. With the demise of wired headphones, especially for high-end phones, this is unlikely to be viable.

that is insidious. Hopefully there is an alternative dangling a charging cable or someone coming up with an antenna case that can boost the signal. So far I'm good with just not buying one like that.

Because few people already carry FM store radios in their pocket, so relying on them requires solving a logistical problem.

True, but the FM solution could really be low latency, unlike the digital rube goldberg machine equivalent.

It will be more reliable than any solution based on either (i) a battery-powered web server, or (ii) mobile internet.

And latency will be constant and near-zero.

I suspect that's why a couple of the use cases you mentioned (city tours, dance parties) are usually addressed with FM radio and not apps.

Perhaps there's a way to do this with a modern bluetooth stack. But none of the offline messaging apps I've seen (Bridgefy, Meshenger) support group audio.

What's wrong with the reliability of mobile internet? I'd expect that in the target environment (e.g. some building in the middle of a city) mobile internet would be like 100% reliable, that would be a problem only in remote wilderness areas.

The things I had in mind:

1. I sometimes participate in Zoom/Meet calls when walking around SF. Not often enough to have a representative sample, but I know I've had a non-zero number of 1-2 second drops.

2. Students are often on a budget, and may have data caps on their mobile plans. Running out of data towards the end of the month might be OK if you're mostly in your room or the library. But it might impede you from participating in a lecture if you're walking out of wifi range.