That seems to be the default translation. But your emphasis on "the" suggests you had to translate "Dolphins eat small fish". That is a general statement about all dolphins and the way to express that in Romanian is using the definite article. So "delfinii mănâncă pești mici" can mean either "the dolphins (some particular dolphins) eat small fish" or "dolphins (all of them, in general) eat small fish".
The audio sounds like "Delfinii mananca pestii mici" ... that is, "pesti" would have a barely audible "i" sound, where "pestii" would have a clearly audible "i" sound. In this audio there is a clear "i" sound.
Further there is another sentence on the same selection: Rechinii mananca pestii mici
Same sentence, different animal, so why do they insist that it is "pesti" and not "pestii"??