Not sure if this has already been mentioned, but.....

Just another wild theory of course.

in which part of the episode is this?

In Silence in the Library she had a distinctly South African accent when she said Professor Riversong Archaeologist.. Now it has become English.

No, she didn't. It was an English accent.

As "The Papal Mainframe" was apparently the Big Boss of the Monks, it's unlikely that she would be making announcements over the PA system, don't you think?

I suppose River might have got a job as announcer to the Monks/Clerics, but as she's a Professor of Archaeology, it's unlikely.

Unless no one would employ her after her time in Stormcage, of course, so she was doing it to earn a few bob.

On the whole, though, and after much thought about the likelihood of it being River's voice, I think it's not.

I don't know really, it seems reasonable that the Papal Mainframe would talk to the Clerics and/or Monks. I can't remember what the announcement was, but it wasn't asking one of them to move a car that was blocking an entrance

Anyway, it did sound like River's voice I think. Which is odd.

It did not sound English when she made her announcement. LATER it became distinctly English. Maybe Alex Kingston simply forgot. She has played South Africans in films.

The papal mainframe was talking to The Fat One, asking if he was ready to make a donation (about 7:14 - 7:31 on iPlayer).

It doesn't sound much like River. Deeper, and the pronunciation of certain words is different (as though the actress narrating it has had to disguise a regional accent).

I didn't get South African. She was smirking as she said it - perhaps her smile distorted it.

And why would she only say the introduction in South African, but then immediately afterwards speak with an English accent?