It's on every sentence for languages which have a text-to-speech engine implemented. There is no compatible engine for Irish (and a few other languages), necessitating the hiring of a voice artist.
The benefit of this is that (at least now that the new voice is implemented) the pronunciation is more true to life than in the other courses using the TTS engine - however, the downside is that as the voice artist has to be paid per sentence, the cost of this simply eliminates the possibility of having every sentence voiced.
From what I've read, however, every word is pronounced in at least one sentence in the course. It's unfortunate, but it's just one of the necessary adaptations that must be made in catering to certain languages.