You will need to keep them in sync manually because the audio and the video will be running on different clocks. Normally you will want to sync the video to the audio so that the audio doesn’t drop any frames.
There are some callbacks you can use to help determine when the event starts playing and you can also get the current timeline position of the playing event.
http://www.fmod.org/questions/question/i-have-separated-a-video-clip-from-its-audio-how-to-make-sure-they-play-in-sync/