After all, I've recently encountered a couple of advocates of the papacy who argue that, although Augustine didn't say "Roma locuta est," he did say "causa finita est" (the cause is ended). This is true.
Here's the relevant portion from Sermon 131 in context:
Although he did say "the cause is ended," this sound bite doesn't actually help the papal advocate, for at least the following three reasons:
For already two councils have, in this cause, sent letters to the Apostolic See, whence also rescripts have come back. The cause is ended: would that the error might some day end! Therefore we admonish so that they may take notice, we teach so that they may be instructed, we pray so that their way be changed.
1) The appeal is to settled conciliar authority (not papal authority as such). So, "Rome has spoken, the case is closed" is not a very accurate summary. A more accurate summary would be "two councils have spoken - the case is closed." That's not to say that the rescripts weren't from Rome - they were.
2) The reference to rescripts is a reference to a response from Rome regarding the decisions of the councils. Such a rescript neither has its own infallibility nor gives infallibility to the decrees of the councils, whether considered by Roman standards of that day or this day.
3) Notice that there were two councils, not just one. This is part of Augustine's point. His point is that, in terms of church court process, continuing this debate is beating a dead horse. He's not saying that two councils is a magic number, just as he's not saying that getting a response from Rome magically makes the conciliar decisions correct.