The only GUI option at this point is phwhite’s 3rd party module, Text to Speech:
It worked fine in FreePBX 11, and I believe it will still work in 12, but haven’t tested much. You won’t be able to use it directly in an IVR, you have to create a system announcement and then go to the IVR. Requires manual install from the repo and it will trigger unsigned module warning if you install.
edit
The above module is broken for FreePBX 12.