ع not explained
It appears - and full on - in lesson 6 ("Description 2") and yet first explained in tips and notes of lesson 10 (phrases!)
How is that a good idea? ع is difficult enough to understand even with the explanations, but without them?!
Hamza is also explained belatedly two lessons up the tree. I get it that it`s some sort of philosophy, and I even commit to it in general. But there are topics that should be treated differently.
OK, so I ended up watching a few videos and it seems ridiculous now the idea of ayn I had before based on hearing it here alone. It just so completely different. I think this one can be recommended, though possibly there are even better ones somewhere, of course: https://www.youtube.com/watch?v=Y0ro6b50-Lk
It is just a letter, if you know how to pronounce it then you know everything. How? It is a vowel. Where? Wherever it is used to spell a word... It's a matter of convention like all other words. Why? Because it represents a sound, like any other letter, that forms a word.
I'm not trying to be sarcastic. Years ago I took an Arabic lesson from a Palestinian native. I already knew the Arabic alphabet for the most part from a different language, but I didn't understand usage or pronunciation in Arabic. I warned, "I'm probably getting this wrong. I don't really understand it, I don't know if I'm pronouncing it correctly." To which she responded, "no.. that's it. It's just a letter."
For some reason, ع confuses people, and I really don't know why. It's literally just a vowel like any other. You don't need to have a vowel explained, it just represents a sound. Arabic is unusual in that it has so many different vowels and tends to use far more vowels than most other languages, which tend to use one vowel between consonants. But if you're asking about a general grammar rule related to ع that explains how/where/why, there isn't one. It's literally just a letter like any other.
EDIT: Ayin is considered a consonant. I don't know why, I don't make the rules.
The computer generated pronunciation is not accurate. I wouldn't trust that what you hear from it is a perfect pronunciation. Also, although Arabic is more consistent than English, I would assume that there are still some exceptions. It is impossible to explain "why" a letter does what it does with perfect consistency in an organic language.
If still in doubt watch this vid - she gives examples on how the words sounds and they are written on screen so you will find words with ayn + alif there as well https://www.youtube.com/watch?v=Y0ro6b50-Lk She write in Latin, unfortunately, but you still can figure out what the sounds are.