copy right on sound effects in iMovie

I just got a strange email from youtube. They claim a movie clip of mine contains material which possibly is the property of "GoDigital MG For a Third Party".

I only used sound effects and music from imovie. Isn't this material free to use and publish in my movie clips?

Does anybody know more about this? I cannot find any indication of the copy right inside imovie.

It also states that I don't have to do anything, my clip is still available world wide but they could advertise on my page.

Thanks for any help in advance. If this meant trouble I would remove the clip. Its only a holiday film. Or I would move to Vimeo.

Cheers

Chris