I think it depends on the individual viewer. For me personally music is more important than the video in some cases. So I will take for example City of the Living Dead(as it is the first film that I popped into my head) I like this film very much, and admit it has some great visual scenes. But for me without the soundtrack there would be much less atmosphere and suspense.
To take the topic away from horror, I will strengthen my point.
Once Upon a Time in the West: This film has some of the most beautiful music I have heard in film. The emotion this creates is epic,; I get choked up every time
without music this film would be a tad boring for me.
Same can be said for The Good ,the Bad and the Ugly. Especially the stand off at the end, which is one of my favourite scenes in any movie
So when thinking of any great movie like; Superman, Starwars, Inception etc I always think of the music first.
On the other had, there are great movies such as The Dark Knight, which is superb and also one of my favourites. But right now I can't even remember there being any music in the film. That's how powerful the images and story are in it.