We are developing e-learning modules using Articulate Storyline. Currently we are using one of our team to do the narration. My concern is that, as we update the courses on a regular basis, if that individual is not available then we either have to re-narrate the whole thing or have different voices in different modules (not very professional!)  

We have looked at various text to speech software systems to produce computer generated audio with various levels of success, but has anyone had any positive experience with this in the e-learning field? I want the output to sound as professional as possible.