What is the correct pronunciation of Xubuntu?
The first one... Zoo-buntu!
That's how it's described here:
Xubuntu (Pronounced: /zù'búntú/) is an official Ubuntu-based operating system.
Lubuntu = Loo-buntu
Kubuntu = Koo-buntu, though I always seem to pronounce it as more of a Kuh sound.
Pragmatic answer: don't forget that these distros are used by people whose native language isn't English... any pronunciation which is not ambiguous/misleading for your audience is fine. Your audience should be able to infer the spelling from your pronunciation. Pronounce "Zoo-buntu" in front of French people and they will google for Zubuntu after the meeting...
Concerning Xubuntu, I go for ksoo-buntu...
My suggestion for Kubuntu would be Q-buntu. I pronounce Lubuntu as loo-buntu (with a short oo, unlike the toilet).