That's possible because your computer sends airplay streams to all airport expresses at the same time.

However, you can't really do that with a phone, since it eats battery and needs a stable wifi connection.

AirPlay 2 solves that by allowing the phone to buffer minutes of audio to one speaker, which then distributes that buffer to the rest of the speakers.

And the other protocols, like Spotify Connect, Chromecast etc, just send the URL of the stream and make the speakers themselves handle it. It's such a better approach that I'm baffled AirPlay 2 still has not adopted it.

Doesn't that mean speakers open a separate request, consuming double the bandwidth.

I just remember that chromecast's protocol was actually unusable on a slower connection.

How does this work if the stream isn't a URL? I often use AirPlay with files on my password-protected SMB share.

It could fallback to the current behaviour just fine.

Of course AirPlay and AirPlay 2 support this, if the application sending audio supports it.

The problem is synchronizing the streams, not the bandwidth used by an MP3.

Original AirPlay (which I'm pretty sure, encompassed a bunch of "versions" over time) had a fixed 2 second lag and was designed around wired ethernet playing long form audio/video. AirPlay 2 seems like a well-needed update even if the user-facing feature list isn't impressive.

Not multi room synced audio.

But there was an app called Airfoil that did this.

I remember trying to use that. I got random audible speed ups and slow downs as it tried to get different speakers in sync. It would never truly blend to feel like a single audio stream.