Absolutely! Silent disco still requires impractically expensive rental hardware to work well as far as I know. A lot of them run off FM radio, since it's the simplest way to go, but nobody owns portable radios anymore.

An OSS app with the ability to sync everyone up over mobile or wifi, on Android or iOS with BYO headphones, would be incredible. This should be a thing :)

I wonder if something like this (without the OSS part) doesn't already exist. Some cinemas in France have some kind of app for people who are either hearing or visually impaired which allows them to follow the movie.

I've never seen in action and don't know how it works, but at least for the audio part it should be able to synchronize the phone with the cinema screen.

If I'm not mistaken, it's provided by this company: https://www.twavox.com/en/

Roku sticks allow this for t.v watching, via the Roku app. No idea how well it works for audio or more latency sensitive applications.

Snapcast has a webapp and a native android client. Although I'm not sure how well it handles many, many clients. In theory, if all on the same WiFi they should all play in sync like a silent disco (at least for those not using Bluetooth headphones where the playback latency is too high/not available).

Web radios handle many clients. The first problem could be if the Wi-Fi hot spot can handle that many clients. The second one is that web radios and their protocols usually don't care if two clients are not in sync. They are usually in different places, maybe different continents.

I'm self hosting a web radio for my LAN at home. I set it up years ago, I'm not there so I can't check the details but I think it is: Icecast2 on an ARM small server with DeeFuzzer (sp?) to send my mp3s to the Icecast2 server. MPV or VLC to play music on my Linux laptop and Transistor from F-Droid (I believe)

They handle many clients but they absolutely do not play in sync. That's never a requirement for them and I'm not aware of any web radio protocol supporting that feature. Web radio is not the right solution for a silent disco type situation where you can at least guarantee everyone is relatively local.

I think wifi + WebRTC would take be too hard for this honestly, maybe add wifi6 as a requirement because it can theoretically have lower latency.

"Their own source" looks like they are bringing their own files or (more probably) their Spotify or YouTube. It happens all the time on public transport. Or did you mean bringing their own music and taking turns at sharing it with the other people around? That might be against the terms of service of some services.

Surely, since "silent disco" only really works if everyone is dancing to the same music (which is the only thing that would make sense for a post about synchronizing audio), they're using "source" to mean "device"

I believe the syncing won't work when playing with a bluetooth device