I wouldn't say "Good evening" and "Good afternoon" are interchangeable in English, other than maybe a fuzzy area between 4pm and 5pm, but in Danish, eftermiddag is used between 1pm and 5pm and then aften is used between 5pm and middnight (or until one goes to bed)
The first syllable is "vel" like in the English "velcro". The second syllable is "kom" like in "comma". Then a second "m" sound is appended:
"vel-com-m". Note that this is just an approximation, in reality the sounds are a tiny bit different from the English sounds.