They should sound the same.
"Hun spiser eple" would mean "She is eating apple" which would be an unusual thing to say in English. You'd usually need to have 'an' before it, just as you would have to say "Hun spiser et eple" in Norwegian for that to be correct.
I think "She is eating apple" is a valid English sentence only if "apple" is serving as an adjective, e.g. "What kind of pie is she eating?" "She is eating apple."
No, that's grammatically incorrect in English. You would need to put an indefinite article before 'apple'.
For the other words like gutt you add en to have the in front, so why does eple just add a t
If the "en" or "et" is in front of the noun, it means "a." If the "en" or "et" is attached to the end of the noun, it means "the." So:
gutt = boy
en gutt = a boy
gutten = the boy
eple = apple
et eple = an apple
eplet = the apple
(The "e" in "et" is dropped in the case of "eplet," because "epleet" would be kind of silly and would change the pronuncation.)