Yeah, the audio needs to be fixed, especially for "rechinii". It sounds like "răchinii".
But you can figure it out just by looking at the words. You cannot have indefinite forms with plural nouns in Romanian (same as in English, German or Dutch as far as I can tell). For example "un rechini" (a sharks) is not correct in Romanian (and English).
In the case of definite forms, it's a bit tricky in Romanian because it is not that obvious at first.
For example, take the word "copil" (masculine):
- copil = child
- copilul = the child (the definite article here is "l", connected by the rest of the word by "u")
- copii = children
- copiii = the children (the definite article is the last "i", the second "i" connects the definite article with the rest of the word)
And now for delfin (masculine):
- delfin = dolphin
- delfini = dolphins
- delfinul = the dolphin
- delfinii = the dolphins
Thank you for the explanation; however, something else is unclear, not what you explained. In English, it is pretty easy to have '(some) dolphins or (some) sharks' without definite articles. It is not clear whether plural nouns are definite, especially when you just listen to the words, not see them.