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For now, Amazon does not allow Echo to do unprompted notifications. It may come as a future update.
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There are several approaches for announcing messages.
A. Similar to the Sonos, the Fabriq speaker has DNLA capability to act as TTS for Smartthings
https://smile.amazon.com/FABRIQ-Portable-Bluetooth-Speaker-Amazon/dp/B01LYZAWRO/ref=cm_cr_arp_d_product_top?ie=UTF8
B. Use an Android device (5.1 OS) with LANnouncer software and BigTalker
C. Use a dedicated doorbell that can play MP3 sounds
There are more options but that should get you started.