Been doing a lot of testing lately as things were very flaky going to my echos. Here is some things I discovered.
If I use the standard notification app things are very inconsistent. Not all the selected echos will respond. And sometimes one of them will repeat the message several times. I tried selecting the echos in both the text and audio device selections. Same thing in both.
I created an app to test with. Found out that using playAnnouncement works pretty much all the time. Had some issues with playText and speak. Not sure what command the standard notification app uses.
If the notification to the echos is triggered automatically, using my test app, things seem to be fine. All the echos respond within a second or 2 of each other. If I trigger the message using an Echo, ie have echo turn on a switch when I tell it to which then triggers the message, the echo I used to tell it will not play the message. All the others do. For example. I tell echo 'good night' which sets a virtual switch in HE. HE then, along with other things, sets the mode to night. I have an app that sees the mode change and announces it on the echos. But the one that I said good night to doesn't.
Don't know if this makes sense or is of any help but just what I've been finding out.