My answer (to audio), "νι ανανάς", was not accepted, and I have reported that, as I did similarly on another exercise where "α ανανάς" was not accepted. Personally, I don't object to these exercises, but there does need to be consistency in the form(s) of the answer(s) that are accepted, and particularly so when one's input is aural (no visual clues). And clearly, both the letter name and the letter itself need to be accepted as valid.
To whomever may see other examples of these oversights, I would recommend that you report them also, so the kinks can be worked out. Once they are, I think they'll prove more useful and less irritating. After all, this skill is all about proficiency with the alphabet itself, so that practice is a highlight, but learning a few words along the way is also good, and helpful. An exercise is better if it can offer a greater degree of complexity, because that keeps one on one's toes and relieves tedium, so I think the idea itself is good.
I agree with comments below. I was asked to translate what sounded like "nee ananas". I responded with "ν ανανάς" which was marked incorrect - "νι" was requested. Later in the lesson, I was asked again to translate what sounded like "nee ananas". Trying to learn from the last "mistake", I tried to respond with what I thought was being requested with "νι ανανάς" but this was marked incorrect as "ν" is what was required. I believe this is very confusing and discouraging for the learner. I suggest this beginning section be modified.