I think we should never assume thats the case....
Personally, I watch a great deal of videos without sound and only switch the sound on if there is likely to be a) verbal instruction or b) contextual environmental sound that I think might be relevant and add to my understanding or enjoyment. A great deal of the time I would rather watch and enjoy/analyse without music. I used to work with numerous people with the same take on this. Also, as been said in these forums, (and where I used to work) music is such a personal thing that we risk alienating an audience through music choice alone, that sometimes it is better to leave it out altogether (not every time, but you get my point)
(Maybe you could conduct a poll here?)
