It must be the duolingorobotladyvoice's fault. It should be 'O' everytime not 'U'. In hungarian language the letters are always pronaunced the same in every word. We have 44 letters and if you can pronounce them correctly then you can pronounce every hungarian word as well. I think it's the same in polish as well.
Hungarian has morning, day, evening and night. No good afternoon. So, most of the daytime is covered by "jó napot". You don't need to wait until noon to start saying "jó napot". Anywhere after breakfast until late afternoon is "jó napot" time. Let's say "regular business hours".
"Almost always." There are certain cases when one of the consonants "falls out", or when 2 consonants staying next to each other changes to an other one (in the pronunciation), etc..
Anyway, if you pronounce every single letter as it is pronounced in the alphabet, everybody will understand it, even though it might sound a little strange.