Ok, thanks for your help. I'll have to see if there's a way of using a different tts as espeak is very robotic. I'm calling the command from RPi's wolfram program at the minute so I'll look at mathematica.

The "a front end is not available" error typically occurs when you try to run a command that requires the graphical user interface. Are you calling mathematica or wolfram? You need the former.

Are you asking M to speak the words "The dark knight" or the output of WolframAlpha["The Dark Knight"]? I suspect you don't want the latter; however your function runs as expected in my RPi.

Never used Pico TTS so can't say anything about it.