I think what I got from it is "ai" would be pronounced /a/ if the "i" was just orthographic like in the example "Caileafóirnia", where "i" is just there because of the slender "e" that comes after.
In "cait" there's no such vowel, so I just assumed the "a" was the one who was merely there for spelling reasons (that completely fly over my head). So, since "ai" is not a diphthong and there's no slender vowel to justify the "i", by process of elimination I would go with /kit/.
Now if that's the case, I would really like to know what the hell that "a" is doing there, instead of simply writing "cit". :)
Melhael: The "a" is there because the basic word is "cat" (cat). In the plural the final consonant is palatalized [made 'slender'] -- /t/ becomes /tʲ/ -- indicated in writing by the insertion of the letter "i". Had the word been written "cit" then the initial "c" would have been palatalized too ...but in "cait" it isn't.. It remains 'broad', as it was in "cat".
Yes, I could hear the audio example before I clicked the on comments. When there is audio available there is a loudspeaker symbol on the left of the sentence. Sometimes when my internet connection is slow (or Duolingo's servers are overloaded, maybe) then the audio is not played at all or played with a delay.