goedendag, goededag and goeiedar are all correct. In the course of the years/centuries the Dutch have dropped the n in speach in many words. This even happened with verbs. Officially you should pronounce the n of werken (to work), but a lot of Dutch just dropp the n (or pronounce it only slightly). Goedendag is like ancient Dutch. Nobody says that anymore. It's goededag (which sounds a bit posh) or goeiedag, which you will hear mostly used by the common people.
In recordings here and in another app, it sounds like sometimes when there is an -en at the end of the word, and also here in the middle of the compound word, the "n" disappears, just sort of getting deleted when it isn't stressed and winding up with just a schwa. Is that a trick of my ears, an issue with the recording, or is it an actual thing that happens in speech? It would be a natural thing to happen, very believable that it could be a thing, just not sure if I should trust what I'm hearing. :)