"I don't like the airplanes."
Translation:Cha toil leam na plèanaichean.
Read the tips and notes to lessons, it is explained in them. The tips are available in the web browser version of Duolingo (unfortunately not in the mobile app). You can also access them on the duome.eu website.
As for na – it’s the plural article, so na cait the cats (many of them), na plèanaichean the planes, etc. But it’s also feminine genitive article: na h-Alba Scotland’s, of Scotland (lit. of the Scotland, Alba Scotland is a feminine noun), na màthar of the mother, etc.
an, am, and a’ are used only in singular number, depending on the noun’s gender and its first sound.
I believe it should be /xa/. There are comments under other sentences (like in "I do not like pepper at all." – I think I saw more under other discussions but now I cannot find any) with similar remarks about pronunciation…
I cannot play those sentence from discussions (as none of the words in the sentence are linked to their vocab entry where you typically can listen to the recorded sentences), so I cannot comment on the sound myself. But if it sounds off it might be worth to use the Report button under exercises to report something wrong in the audio?