I have a two learners experience this issue. In Chrome, when they reach the slide before audio files they freeze, they can go back but not forward. But, when they try the lesson in IE and/or Edge, it won't start. Both of these learners have been able to view other lessons in IE, Edge, and Chrome with all kinds of audio, video, and various interactivities. Any thoughts on what might cause this kind of freeze and crash in browsers?