Voice stopped functioning part way through ride

Hi

I've used the iPhone app twice now. One circular route, once on the default direction, then once with it reversed. Both times I had the screen off and navigated with the voice instructions alone. First time it worked brilliantly. Second time the voice instructions stopped half way, after taking a short break (a couple of minutes), at the destination the loop was built around, and didn't resume for the rest of the ride.

Opening the app showed the route still displayed, and my position tracking along it. I tried playing audio from a different app; that worked (suggesting it wasn't an issue with bluetooth / headphones).