Translation:The cat opened a lot of gifts on its birthday.
Have you experimented with different hardware? Visit a library if you need to. Try using a different computer.
Except for the "s" at the end of muchos, I have no difficulty with listening to the audio file (soundtrack). In the future, my experience with the audio will (eventually) become obsolete information for you to read (at some point).
Yes, it would. Take a look at this link to la gata.
El gato abrió muchos regalos en su cumpleaños.
― The cat opened a lot of gifts on its birthday.
― The cat opened many gifts on its birthday.
If Duo doesn't accept this alternative, then we need to report it.
Please read my reply to glieke.
Are you sure that your sentence contained no errors? The word, his, was not the error. I realize that you were presented with a sentence showing you "...its birthday." But this response by Duo was not triggered by your decision to write "his birthday." I mean if everything was working correctly, this would not have been a reason to reject your answer.
HerczogMri, if Duo's Spanish sentence had been El gato abrió muchos regalos en su fiesta de cumpleaños, you would be correct in saying "at his birthday party" in English. But, as LonzCat says, the translation of Duo's actual sentence (El gato abrió muchos regalos en su cumpleaños) requires on.
I think using correct prepositions is one of the hardest parts of any language--I'm always messing them up in Spanish! And, when to use "at" and when "on" is one of the hardest parts about English prepositions.
Perhaps a very rough (because there are sure to be exceptions) guide might be: If you go to something (like a party or a concert), you're "at" it while you're there. If it is simply an occasion (like a birthday or Christmas), things happen "on" the occasion.
EDIT: Also, try this article-- https://learningenglish.voanews.com/a/learn-prepositions-in-on-at/3073690.html It definitely sounds more like the writer knew her subject.