‘Afternoon’ is deceptively difficult to pronounce due to its stress pattern and vowel sounds.
Firstly, it uses a shifting stress pattern. That means that depending on the words after it, it could be stressed on the first syllable, or on the third:
Stress pattern 1 should be used whenever ‘afternoon’ is said alone or at the end of a sentence/unit. EG “Good afternoon”, “What a lovely afternoon!”, “Shall we meet this afternoon?”
If ‘afternoon’ appears before another strong/content word, the stress shifts to the first syllable – stress pattern 2 above. EG “Let’s have afternoon tea.”, “This afternoon’s weather looks terrible.”
Secondly, the vowel sounds can cause problems. The first two vowel sounds are central /ɑ:/ then /ə/, the final sound is /u:/. The ‘r’ is silent in the middle.
So it should sound like: /ˌɑ:ftəˈnu:n/ or /ˈɑ:ftənu:n/.