Hi. Pronunciation is a bit of a thorny issue since there are a lot of different pronunciation schemes in use, so I’m not sure if you want like the popular pronunciation used by most people or the pronunciation that the Greek themselves used in Ancient times (as far as we can reconstruct it) or the pronunciation Greeks today use and so on and so on…
In Classical times, it was pronounced either as [zd] or [dz] (I believe scholars think the former is more likely for Classical Athens), and later was pronounced just [z], so none of those are really wrong. I think most people on these forums use [zd] and this seems to be the recommended pronunciation in most of the newer textbooks I’ve seen.
Iota- The examples given are pin and machine. Is this sort of a “You can do it either way you want/whatever you’re comfortable with”?
Here’s it’s just that certain iotas are short and others are long but the spelling doesn’t distinguish the two, like it does for certain other vowels. So pin is for short iota and machine for long (although, the two "i"s in English are also slightly different vowels and don’t differ only in length, while the Greek vowels only differed in length.)
Xi- My guess is “kss”, but I’m probably wrong
No, that’s right.
Omicron and Omega- What’s the difference? Is it that Omega is shorter?
Omega is the longer one (you can remember it because micron means “small” and mega means “great”). Omega was also more open.
Upsilon- Is it pronounced “oo”?
Upsilon was pronounced like French u or German ü (and there are short and long versions like with iota) – there’s no real equivalent in English. If you’re not familiar with that sound, the way I was taught it in school was to round your lips as if you’re saying “oo” but then try to say “ee” without moving your lips. It feels awkward at first but eventually it becomes natural.
The omicron-upsilon combination is usually pronounced “oo” though.
Chi- German “buch” I don’t know
In the Classical pronunciation chi was pronounced like the “k” in “kin” and then kappa was like the “k” in “skin” – which are different, but since the difference is hard to get used to at first, lots of people pronounce it using the pronunciation it got later, which is basically a rough “h” sound – it also occurs in words like Chanukah or (Scottish) loch.