Annotea for multimedia documents

Hello,

I am looking for an annotation system for adding annotation to multimedia 
documents and I wonder if annotea may be suitable for this task. The 
protocol specification only discusses annotating HTML documents and uses 
XPointer for the <a:context> property. Why is this defined in the protocol? 
Is this this property restricted to XPointer or may I use also another 
method instead (e.g. timestamp for video)?

Am I right, that on the other side it's also possible that the Annotation 
itself may be something different than a XHTML document, e.g. an audio 
file?

Thanks,
Daniel