Actually, in most places in Norway, the people would say 'Ei jente' meaning 'A girl' and 'Jenta' meaning 'The girl'. The main place where the people would say 'en jente - jenten' is in Bergen, Noway's second biggest city. Both forms are valid though, and knowing which gender to use, can be very hard for English speaking people, as it isn't always logical. This course will be quite a bit easier as it operates with only two genders instead if three.
No, that is not correct.
The conjunction 'and' is always pronounced with a silent g in Norwegian. However, the adverb òg (equivalent with også) can be pronounced like in the sound clip. You can also pronounce it with a silent g. The accent is optional when writing it, which means that both words are often written the same way.
Jeg og han går tur. -- always silent g
Jeg går tur jeg òg. -- optional silent g
See notes on pronunciation in the dictionary: http://www.nob-ordbok.uio.no/perl/ordbok.cgi?OPP=og
The top of the page says: Translation: A boy and a girl. If you wrote that but it still got turned down, then it might be a bug or maybe a typo in your answer.
Én and en are much like og and òg as I explained above: They are different words, but the accents are optional, which can lead to some confusion.
Én is the number one. En is the article a.