It could be missing Codecs but....
First of all, I would download and install
VLC Player. It's free. As usual, with any install, choose 'Custom Install' (if offered) and choose not to install any tool-bars or 'make me defaults'. They are a pain in the hole.
If you get a black screen in Sopcast, try watching it via VLC. Instructions about half-way down the page
here. Use the 'Watch Sopcast via VLC automatically' bit.
If you are still getting nothing, post back and we'll try a different tack.