Basically you want to add a soundtrack to a slide show, rather than adding a file. Unfortunately, I see no way to do this with applescript. Usually you do this manually in the Inspector-> Documents-> Audio section. I also do not see the applescript command for the Share-> Send To-> YouTube command.
It seems you cannot do what you want with the keynote. Keynote does not have these applescript commands. It does not have any keyboard shortcuts for these things, so this cannot be achieved with GUI scripts. Unfortunately.
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