It’s not easy, but you can get echo to do announcements in one of two ways.
First, you can take advantage of the fact that the echo is also a Bluetooth speaker and use an android device like with LANnouncer mentioned above, and you can get your notifications to play on your Alexa devices.
( as you’ll see in the following thread, when this was first suggested I didn’t think it would be possible , but I turned out to be dead wrong – – the guys got it to work. 
)
Or, you can set up askAlexa. That one requires that you sign up for an Amazon developers account and again require some technical expertise to set up, but you can do a lot of magic stuff with that. 
http://thingsthataresmart.wiki/index.php?title=Ask_Alexa
Personally, I am waiting for Amazon to add an official TTS feature, but if you’re willing to do some work you can have it now. 