Randomly the accompaniment will get REALLY quick and the count 1234 sound like a chipmunk (really), but the bar to follow the song goes to a normal speed => stop/start on uninstall/reinstall => work.
Almost all problem are solved by stopping/starting the app (stopping=swap out), or by uninstalling and restalling the app. The only inconvenience in uninstalling/reinstalling the app is that you have to redownload the lessons you want to do, but you lose no progress.