It could be a browser issue?
It could also be an internet issue for the user?
The other is that if the are long mp3 files, the server times out the request.
I would log a support ticket with BC and be as specific as possible to the nature of the issue and if there is a way to replicate it. When the Student / Client tells you about the issue it would also be good to get there browser version and platform details to try and create a replicable error that can be solved.