If you get the error message "Registration failure" when you try to use the speech engine in FL Studio, follow the steps below.
1. download this registry file.
2. when asked, save it to your computer
3. open Windows Explorer and go to the location where you downloaded the registry file
4. double-click the file to add it to the registry
5. answer yes when Windows asks you if it's safe and if you want to add it to the registry
Afterwards the speech engine in FL Studio should work.