Translation:The girl heard a dog in your house.
No because "the bell" is an object, not a person. Therefore it would be "La niña oyó la campana." But if she heard her brother instead, than it would be: "La niña oyó a su hermano." So you put "a" after a verb when the object of the verb is a person or a pet. Sorry for the confusion.
Google is malfunctioning when I try to search for a ratio of results for fragments of the sentence with and without "a". But "oyó un coche" gives 34,900 results and "oyó a un coche" gives 9 which suggest the "a" in duolingo's sentence is the personal "a" because the dog is a pet.
Native speakers of Español have many different accents, but generally I have found that these speakers, to an anglophone, confuse j with y, and b with v.
In Colombia my friend would constantly have her name spelt with a y. Jennifer = Yennifer
In Spain I hear "bale" rather than "vale" from many people and this is reflected in their spelling, just as I see some of my Colombian friends writing cilla rather than silla.
In response to your question, specifically, y can have a 'ch' ish sound. So expect to hear:
cho = yo ocho = oyo …etc
Yes. Almost all Spanish speakers outside Mexico pronounce the "y" like a consonant when it is in the middle of a word. It's typically like a "soft" "ch" sound, but in places like Argentina, I believe they pronounce it like the "s" in "leisure."
In fact, I know many speakers from South America and Spain who think pronouncing "y" like the English "y" sounds "ghetto" and "uneducated." This means a hell of a lot of native Spanish speakers look down on standard Mexican pronunciation.
Note that on TV, Mexican news announcers typically pronounce "y" the way they do in Spain.
This is similar to in Japan, where the /g/ sound is pronounced differently than a lot of Japan does. It's also similar to how many people in the US think Ebonics sounds uneducated.