Is this possible to detect not-Windows-included TTS-engines?

[perdolka]

Like "Acapela Elan TTS", "Lernout & Hauspie" (L&H) etc.

[ettoolong]

At this moment, the answer is NO.

TTSFox is base on this WebAPIs :
https://developer.mozilla.org/en-US/docs/Web/API/SpeechSynthesis

It's depending on browser support.