It's sort of irregular, but it also follows a common Germanic language pattern called umlaut, where the a sound becomes an e sound, o sound becomes ø, and u becomes y. It shows up in nouns, but also in other word classes, like adjectives and verbs. Compare with Englis man - men, bad - better, foot - feet.
No, but it should be pronounced as enn - ner, while in the audio it's pronounced e - nner
Some people do, but the standard pronunciation omits it. In my dialect, which is a western Norwegian dialect, we do pronounce it.
There isn't any irregular form for nouns in plural in my language (portuguese).
It's weird that english and norwegian has O: Never thought about it.
Is the change from 'a' to 'e' a rule that applies for all nouns starting with an 'a' in singular or is it just an irregular plural?
It's irregular, as Deliciae wrote. I see you are also learning German. The change from 'a' to 'e' is actually the same concept as the umlaut, where an 'a' turns into an 'ä'.