After reading some comments about the pronunciation, I can understand better the pronunciation of một in this particular exercise. I think it is very important to explain this in the Notes & tips section in the future. By the way, the sound of the voice is really good, so understandable and clear, and also I think the final part is a bit accelerated but this makes it more natural and beautiful. :)
It sounds like "mồn". But we can guess from the context. When native speaker read the sentence really fast it may sound something like that though.
Most native speakers I have heard and most Vietnamese songs I have listened to or sung, pronounce "một" as above. Sometimes it is very pronounced, like they are adding another word to it.