I remember a very long discussion - with several Esperantists with decades of speaking experience - about a specific recording. Several heard a major defect with the sound (a buzz, or a gap... I don't recall, but a fairly blatant issue). Others insisted that the recording was fine. Myself, I heard the defect - and it was blatant.
After a long discussion, I tried from a different computer with different speakers and the problem went away. Once i mentioned this, several people who previously couldn't hear the problem, found they could hear it on different speakers or on a different device.
See my other comments in this thread. The speaker is saying "knabo". If you're hearing something else, try a different device, different speakers, headphones... or ultimately, consider it ear training.
This is normal conversational speed. If it sounds too fast, consider it ear training.
In short, "da" is for quantities. How much bread do you have? Kiom da pano vi havas? I read a lot of books. Mi legas multe da libroj. I want to learn a billion languages. Mi volas lerni miliardon da lingvoj.
"de" is for... well, for all of the other times you would use one of these. ;)
This graphic provides a nice visual. The Tips & Notes also cover these in Da/De and again in Food
So it seems like the "correct" translation has to be the one you expect people to use in everyday English. This means if you type "The mother of the boy likes tea." You can expect an error message. Despite this sentence being correct, only Spanish speakers would find it 'normal' sounding. A true native English speaker would opt for the short version of "The boy's mother likes tea."
In a nutshell: both are correct but only the one with a possessive 's is going to be accepted here. Even when another example accepts both translations with the word "of" and the possessive 's.