Text-to-speech won't speak ...

once again this error came out.

this is a very common error. (google search)

usually it is advised that a reinstallation of the text-to-speech speech engine (SAPI) will correct the problem.

install SpeechSDK51.exe

most of the time it won't correct the problem.

the engine itself is intact and it is those TTS voice packages that triger the error.

so Anna, Mike or Mary on the system won't speak.

but Microsoft doesn't offer standalone TTS voice installation packages.

now move on to install the "SpeechSDK51LangPack.exe" on the same download page mentioned above.
(it is similar to the case with reinstalling the Microsoft Reader Speech Engine and Reader TTS voices. Download MS Reader here. (valid until 30 August 2012.)

a new TTS voice ("Sample TTS Voice") is added.
(male English voice)

now the system should speak again.