I don't know if this if the problem, but I've had the experience of
listening to a book with the synthetic voices at a high
speed. Suddenly, the machine locks up. I've discovered that two
things can fix this at least for a while. Resetting the speed by
pressing the #5 key will start the reading where it stopped at normal
speed. The other thing that you can do is to press the play key
which will put it into pause mode, and then press it again. This
will start reading at the speed you already have set.
If this doesn't fix things, I have no other idea. Hopefully, this is
something you haven't tried, and it will work.
Kim Lingo