Oxford commas are not a stylistic option in Norwegian. If you fear you may otherwise be misunderstood, feel free to use one in order to disambiguate, but if that's not the case you should avoid them.
To quote Språkrådet:
Husk at vi setter komma for at det skal bli lettere å lese, ikke for at det skal bli vanskeligere å skrive.
Hover over words to hear them more clearly. During a listening exercise, press the smaller turtle button to slow it down as well.
Additionally, you can click each individual word on the discussion page (Duolingo will take you do the dictionary page) to isolate it from the rest of the sentence and only hear it being pronounced.
The first half of the sentence is indeed slower than the second half due to commas separating en mann and en kvinne.
Know it's normal to feel a little bit overwhelmed when you start hearing full sentences in a language that is completely new to your ears. Time and exposure are the medications you should take.