I managed to create a custom sound set for Outlook 2011 by dragging the custom sound set from my Entourage Sound Sets folder to the Outlook Sound Sets folder in my Microsoft User Data folder.
I'll update this with more precise directions later.
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Make a custom folder under:
User/Documents/Microsoft User Data/Outlook Sound Sets/
Something like "Custom.eragesoundset"
Put your wav files in there and refer to them in a soundset.plist file in the same folder. That file should look like this...