I have just reviewed all the comments on this page. From this I have observed the following
- I have listened very carefully to the sentence on a computer but of course I was not able to go back to the original question. Rather I did as MimiMcC99999 did and found the sentence in the list of sentences, with the option to hear the audio.
- The recording is quite clear, and there is no way anyone could here an m there. It is just possible that someone could hear an i but even that does not seem likely.
- But there are five independent witnesses that say there is an m in thu.
- There is another sentence in the list of sentences, Chan eil mi brònach.
The only possible conclusion I can come to is that the wrong sentence is being played to some people (and we have no idea if this is on the app or the website or both). It is less likely that it is the sentence I quoted above as this has a very clear mi and no a-nis. Much more likely is that a mistake was made in a recording where someone really did get muddled in what they were saying (which is very easy when recording a load of sentences that are very similar) and that what some people are hearing really is a garbled mi/thu sound. Because of the mysterious way Duolingo works I do not know if even the mods would be able to check this. But what is clear from all the comments is that it is simply not possible that everyone is hearing the same recording.
I am using the app and I can very clearly hear an "oo" sound for thu. But if I force myself to listen for it I can see where it would sound like a m or n sound to someone expecting that.
I don't think there is any error in the recording. I think people are letting their own expectations affect their hearing.
In the past this was not standardized, and Duolingo was inconsistent. However, the recent GOC (Gaelic Orthographic Convention) has decreed, with no obvious logic, that adverbials of time, such as this one, a-nis 'now' should be hyphenated, but that adverbials of place, such as an sin 'there' shouldn't. Duolingo is in the process of standardizing. This is discussed here.
In addition, the hyphen causes technical problems for Duolingo so they have to allow the space as an option. D
I'm afraid I have listened to it and it is pronounced very slowly and clearly and is undoubtedly thu to my ears. Remember the th is silent in this word, so you have to judge the difference only by the vowel.
Note than no one knows why it is silent in this word - indeed no one knows why it is a th at all. It is a t in Irish, Welsh, French, Spanish and Portuguese. Only in English, Scots and Norse do you find th, but this is voiced in English and Scots, so would have been written as dh if it had been borrowed into Gaelic. The only possibility is Norse. The word was þú, where the þ /θ/ (like the th in English thing) would have become th in Gaelic, because that is how th used to be pronounced. But why it went silent, instead of changing to /h/ is beyond me.