David,
Let me assure you that you are not the only one with issues with video play back. Video in OpenLP especially on windows is a little bit hit and miss. Usually I get the picture and not the sound!
Having said that, VLC, for me at least, works wonderfully! There have also been some bug fixes for video that have gone in since version 1.9.8. Hopefully this should further improve the situation.
Also as Raoul has said, the coded packs do help. K-lite being my favourite.
God bless,
Phill