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KevinH, DiapDealer - Thanks for the explanation re file types, actually I was surprised that I was able to add whatever I liked, I expected to be restricted to MP3 and MP4. I also hadn't spotted the File->Add->Existing files option that's why I was adding via the Book Browser folders.

I discovered what causes Sigil 0.9.5 to crash, and indeed it is shell extension. But I've had it in the menu for so long (since NT4 at least) I had forgotten it wasn't part of Windows, all it does is to create a
flat zip of a folder's contents - Zip Folder.
FWIW - I reinstalled Sigil version 0.8.4. I think that's what I used when 'playing around' with adding media files to ePUB 2.0 a few years ago. That version of Sigil doesn't crash when I right click on a file in the Add Existing Files->Open File dialogue when Zip Folders is enabled.
However the issue has been rendered moot.
Yesterday my colleague demonstrated a different approach, rather than creating multimedia epubs, use an android application that pulls information down from the cloud, stores it, and presents it on demand. We've decided to go with that rather than EPUB 3.0. But not because of this problem, I hasten to add.
BR