I take Mary's comment to be correct, which is that, if you were asked at any point to sign a consent form, then legally all usage of the footage will default to the production company. But, then again, Aidan, your post gave me the impression that this was done very much *without* any consent...
As to the actual streaming on Youtube, that seems very mysterious...Nathan's guess is possibly the most sensible, but it begs several questions. Have you yet been cast in this piece? Do you know if you weren't cast, and *then* the footage appeared? Were you cast a while ago, and the footage has only appeared now? Or is it just it's likely to have been there for a while, but you recently stumbled upon it?
Naturally, for Nathan's suggestion that what is being done is that the auditions are being streamed via Youtube in order that the clients can see them to be true, the casting would still need to be ongoing (or done at an earlier period before you realised the existence of the material!). At any rate, I don't think working through such a cheap, poor quality medium for casting purposes reflects well on the production setup: if they are really are saving on the DVD costs, then that tells you a lot about the quality of the production company and the casting director they're working with!
If none of this is the case, of course, then I'm not at all certain what the use of this broadcasting of auditions actually holds. It could be some kind of 'vanity' broadcast, I suppose, if they wanted to promote the quality of the auditions, but given that I've had a quick look, seen there are no tags, no descriptions of what's being broadcast etc., it certainly doesn't work as advertising in my book!
By the by, Aidan, how did you even discover this material's existence if they never suggested it would appear on Youtube? Were you just doing what I sometimes do, and googling your own name (or the project) randomly to see what came up?