After a few more tests with GraphEdit...
CoreAAC seems to have some issues. If I build the graph (or modify it) it works correctly in GraphEdit.
If I save the graph, reopen GraphEdit and open the saved graph, I get incorrect behaviour.
If the graph is a simple video / audio render as described in my first post, the video plays quickly, it seems to be ignoring the clock. The only way I can fix it is to modify the graph pin connections. This seems to return the graph to normal behaviour, at least until I save it and reopen it.
If I add an audio filter between the CoreAAC decoder and the Audio Renderer, it also works as expected until I save the graph and reopen it in a new GraphEdit session. When reopened this version behaves as seen in Media Center. That is the video does not play, but I can seek the video, getting still image renders from wherever the seek stops. This behaviour is also rectified by modifying the graph pin connections in some way.
When I substitute ffdshow for CoreAAC in GraphEdit, I get normal behaviour, before and after graph modification or reopen.
So it looks like CoreAAC has issues, possibly with clock handling.
I will report this to CoreCodec. If anyone has any clue as to the cause I would appreciate the feedback.
Thanks and regards,
Dennis MF