With a strong and consistent WiFi signal, you should have no trouble downloading the spoken sentences. We have checked this here and have been able to download both British and American soundfiles. Please do try again.
You could also try force quitting the app and restarting it, or restarting your device, to see if this solves the problem.
The good news is that if a device loses internet connection mid-download for whatever reason, or if you cancel the download, the download will resume from where it was before when you tap to start again, i.e. you don’t have to start the download from 0% all over again.