In some spoken sentences you may hear an extra sound at the end of a word. For example, an "n" sound may be heard with word "một" and you may hear it as "mộn".
This has been bugging me for years as to why. I first heard it in songs, that are usually in the Northern dialect.
It is not a mistake. Today, some native speakers told me it is termed, "luyến láy". I did a Google translate on it that revealed "leaf attachment" but my friends said it is more like "rhythm" in English as they understand it.
If there is a consensus on this, and Duolingo accepts it, I suggest it be added to one of the earliest Tips and Notes to prepare students, rather than have them baffled through the whole course.
Vietnamese is a tonal language. Rhythmic too. I love it.