There is no plural to "pick up" in the audio.
"Il mange une baleine." and "Ils mangent une baleine." are pronounced exactly the same. There is no pronunciation difference. (In theory, you could have liaison between "mangent" and "une", but it is optional here and sounds odd to me.)
And this verbal ambiguity is hardly unusual in French.
- "Il parle." and "Ils parlent." are also pronounced exactly the same.
You do sometimes have a difference between third person singular and plural, either in the verb itself or with liaison between the pronoun and verb, but not always:
- "Il lit." and "Ils lisent." are distinct.
- "Il aime." and "Ils aiment." are distinct.
But add a direct object pronoun and that can disappear too:
- "Il m'aime." and "Ils m'aiment." sound exactly the same.
In conversation, with context, you will probably know what a speaker means. In a one-sentence Duolingo example, you won't.