That's interesting, but I remember dragon shouts are not magic. They can't be silenced.
Silence doesn't just stop magic. Its in a book that silence can be used to quiet the overtalkative wife as well. And since thu-um is the power of voice, if one can not use their voice, one can not use thu-um or a dragon shout.
Silence would be a godly spell if it ends up being that way...it may be possible that dragons have a way to resist silence, but the basics of silence is to make it so that the enemy can't talk. And if dragons can't talk, I think its fair game to assume they can't breath fire or fly.