This app was mentioned in 3 comments, with an average of 1.33 upvotes
You can use an Android device with LANnouncer.
Install the app on the Android device, add the device in SmartThings, send whatever text you want to have spoken to the Android device.
I use an old cell phone with LANnouncer.
I'm using lannouncer (https://play.google.com/store/apps/details?id=com.keybounce.lannouncer) with smart things for this, it works well.