I had the same problem
On the flash site I had to turn my volume to 100% and still couldn't hear - but when I mentioned it in the forum I was told it was probably something to do with the flash settings in my computer. So I tried it on my other computer and the volume is fine - it was something to do with my computer - but I haven't taken the time to try to fix it yet. They also suggested I use google chrome as that is sometimes better for the flash videos.
