Translation:What are we doing in November and in December?
It's there only faintly because of the n's on both sides. The "n" and "k" of "csinálunk" practically merges into a "ŋ" sound (like in the English word "sing"). Hungarian doesn't like consonant clusters, so similar things happen quite often.
If you listen to "mit csinálunk", you can hear that a separate "t" isn't pronounced either. It merges with the "cs" of "csinálunk", so a stressed "cs" is pronounced instead ("miccsinálunk").
I think it's a matter of text-to-speech software being available for some languages but not others.
TTS was not something Duolingo developed; it's an off-the-shelf component sold by other companies, and they don't sell them for all languages in the entire world.
Also, different TTS engines give different results; perhaps there are TTS engines available for Hungarian but none was deemed good enough for the course.
It's something called 'vowel harmony', which makes the vowels of the suffixes match the vowels of the word they're attached to. In this case it's the 'backness' of the vowels that matters, that is, whether your tongue is in the front or in the back of your mouth when you pronounce the vowel.
Front vowels: e, i, ö, ü and their long pairs
Back vowels: a, o, u and their long pairs.
Most suffixes will have two forms, one with a front vowel (like '-ben') and one with a back vowel (like '-ban'). You just need to check whether the word you attach the suffix to has front or back vowels and you'll know which form to use. :)
So 'december' becomes 'decemberben' and 'január' becomes 'januárban'. 'November' contains both front and back vowels, but since the last two are both front ones, the suffix will be '-ben'. The same is valid for 'februárban', just the other way around.