Makkers

Both of them are Bing domains correct. (Not assuming on Googles part) adding "speech.platform.bing.com" as a HTTPS Bypass, Text to speech worked straight away. (After narrowing down Smoothwall's MITM cert was interacting with Text to Speech) Also Found a new articles saying Google doesn't like MITM Certs, Added the device as a exception and they worked instantly so Smoothwall's HTTPS inspection was stopping something. (I suspect they might be sharing resources)

I've just resolved the issue at a few sites, If you add the following domains to the HTTPS bypass "Text to Speech" should start working. speech.platform.bing.com <- Google Text to Speech sr.bing.com <- Bing Text to Speech