As far as I know, the only way to do this is through an Alexa routine. This is a little clunky to set up, but works well, and many community members use this method.
Alexa routines at this time can only be triggered from a sensor, lock, echo brand button, or Flic button. So if you want your notification to be spoken when a physical sensor connected to smartthings detects an event, that’s easy.
But if you want it connected when something else happens, then you need to create a virtual sensor that you can turn on and off from smart things so that that virtual device will start the Alexa routine.
So you can definitely do it, but the set up is more complicated than you were probably expecting.
Here’s the community FAQ: