I recently upgraded to Real Player 11, and to the latest Windows Media Player, after downloading some software so that I could watch Satellite TV and download films. Now existing clips that could previously play came up with an error message.
I downloaded VLC Media Player as suggested by Kelvin in another thread, and now the pre-existing clips DO play, but without sound!! VLC Media Player also successfully plays films perfectly that WMP and Real Player couldn't (mpeg4 files).
So VLC Media Player has ALMOST solved the problem. The clips that it now plays without sound were downloaded from my mobile phone.