It will only respond to a voice-initiated command but it can also act as a Bluetooth speaker and play a custom MP3 streamed from another device. This is a method that a number of community members have used to play sounds or voice alerts through the echo, and is the one described in the first response.
It’s not as elegant as a true text to speech capability, and it does require an additional device, but it is doable.
I was skeptical at first, as a following thread will show, but in fact the method does work.