I'm not a native speaker, but I can tell you that the n at the end of a word, when preceded by unstressed e, is optional. A lot of native speakers seem to never pronounce it, althought it's not wrong (yet!) to do it either.
An exception is when the following word starts with a vowel. This is similar to French, where n at the end of a word is not pronounced but makes the vowel nasalised. Except when the next word starts with a vowel, in which case the n is spoken and there is no nasalisation.
Now we are really getting into territory that I am not qualified to speak about as a native speaker of German who hasn't heard a lot of Dutch. I'll try anyway, and then maybe someone can correct me if it's wrong.
I think this secondary rule may not be as strict as in French, and even there it seems to be losing strength. On the other hand, I would assume that pronouncing the first n in goedenavond is still the standard, and if Duolingo's voice doesn't do it, then it is because it erroneously pronounces the two constituent words separately.
Or maybe it's more complicated and a native speaker can explain?
I wonder whether it has something to do with regional variation, as in the case of the letter 'r': https://www.youtube.com/watch?v=7C8iwl2pNlQ
In a somewhat related note, I have been watching this show to try and make sense of Flemish (Vlaams) pronunciation. Don't do this at home, kids. https://www.youtube.com/watch?v=60-Ptj_Tsxc
R is special in Europe. It was once rolled (in one way or another) basically everywhere. But only a few centuries ago it became fashionable in Paris not to roll it any more. This has started a wave of pronunciation changes for this consonant throughout Europe. Most of Germany is non-rolling already. I think currently Czech is being converted as the first Slavic language.
Of course n can still have regional variations for other reasons.
As a native speaker, I can add to it that the following is existing too: "Een goede avond" naast "Goedenavond" So these things are sometimes mixed up. Than you get the convusion. But the good thing is: everybody will understand you if you use either one of them, or even mixing them ;-)
I am a native Dutch speaker, and funny enough we do not often greet with this word, but when we do, then yes the N is silent, of course depending on the accent as well. Anyway what this word is also a referent for, is a weapon used in the medieval times. Try to find it on Google pictures, you will see what I am talking about ;)
"Goededag" is used all day, but often in the afternoon when "goedemorgen" cannot be used. See SvenDK.
"morgen" or "ochtend" is morning (6-12am)
"avond" means "evening" (6-12pm or really there it is 18-24)'
"middag" means "noon or afternoon" (12-6pm or really there it is 12-18)
"namidag" means "afternoon" (3-6pm or really there it is 15-18 )
No, as Belgian, I can confirm that we use that word at any moment of the day. It is however more common to say goedemorgen (good morning) during the moring, while indeed we would use goedemiddag (good afternoon) less frequently, and use goedendag more frequently in the afternoon. The subtleties of local language usage ;-)
Just like in English, when "Good day!" was still in use:
When you meet someone, it means something like this: "I hope you have had a good day so far!"
When parting it means something like this: "I hope you will have a good day!"
That said, the second meaning is relatively rare in German, and I expect that also in Dutch the first meaning is at least a bit more frequently used since for the second there are more alternatives.