Just a comment
Even slowed down, sometimes I have real difficulties trying to understand what is being said. The subtleties of the pronunciations elude me all too often.
Also, sometimes the slowed-down speech is completely different than the normal speed speech, especially with respect to articles (for instance, the slowed-down speech often distinctly says "una" when the sentence has "un" in it.)
"un" vs "una". In normal speech "una anatra" would sound like "un'anatra". In slow speech you would hear "una anatra".
Sure. But I'm talking about situations where one might hear "un libro" in quick speech, and "una libro". Yes, there are situations where we're just mishearing. There are also situations where it's just clearly wrong. I'm talking about the latter.
The sound recording could be wrong. People have also commented on the poor quality of the recording in other discussions, so it could also be that. I know I sometimes have to guess at what was said. (personally I haven't noticed any incorrect recordings though, just unclear ones).
As far as learning goes - it will help if you know the following rule (one of the very few things I remember from italian classes many years ago): A noun always takes either un or una, it never switches between them. If a noun ends in the letter o, it will require un - eg un libro. If a noun ends in the letter a, it will require una - eg una mela. There are no exceptions to this (except possibly a few imported/foreign words).