Not because it would be impossible, although the "every phone" is a bit of a stretch given how hard it would be to build an exploit that reliably works on all the messed up versions of Android that vendors put out.

But because if you had a capability like this, you wouldn't burn several full exploit chains just to broadcast a speech.

Doing something on the network side (either compromising existing infrastructure, simply being the infrastructure provider, or providing fake base stations) and then simply calling each phone - sure.

Pushing emergency alert cell broadcast messages with a link to the stream - sure.

Actually exploiting the phones? Nah.