Scenario triggers don't match up with the sound in which they are based

Let me explain.
I am trying to depict the Rohirim charge for LOTR 3: Return of the King. I had downloaded a scene which I converted to WEM in order to base the scenario triggers to the sound, which worked perfectly.
The only problem is that I couldn’t remove the other game sounds, such as the swords, maces, bows, ballistas.
So I decided to mute the video sound completely and re-add the same sound from the scene, in which the triggers are based.
At first it matches up, but after a couple of minutes, I noticed that the video is way ahead the audio.
Is there something I’m missing?