Google has limited repetitive use of their free online TTS converter. Google TTS is a great asset but unfortunately they have set limits I can only assume due to usage. This is an example of the web url message:
About this page
Our systems have detected unusual traffic from your computer network. This page checks to see if it’s really you sending the requests, and not a robot. Why did this happen?
This page appears when Google automatically detects requests coming from your computer network which appear to be in violation of the Terms of Service. The block will expire shortly after those requests stop. In the meantime, solving the above CAPTCHA will let you continue to use our services.
This traffic may have been sent by malicious software, a browser plug-in, or a script that sends automated requests. If you share your network connection, ask your administrator for help — a different computer using the same IP address may be responsible. Learn more
Sometimes you may be asked to solve the CAPTCHA if you are using advanced terms that robots are known to use, or sending requests very quickly.
IP address: 7x.xx.xxx.184
I am looking at other options for Text to Speech (TTS), but not very promising for audio quality. I have been testing espeak, it’s better than festival but not by much. Cepstral can be used but you have to pay for licensing.