Using ett implies that you are emphasizing the fact that it is one letter, not two or three.
Ohh, so it is a difference between article and numeral? :) I understand now, thanks :D
Jeg ser ett fjell - I see (only) one mountain.
Jeg ser et fjell - I see a mountain.
I am just starting, but I have a Norwegian friend, so he explain to me, the et it's sounds just like et, and ett it's just like that but longer, like etttt (but you're not say that like what I wrote)
It should be pronounced /breːʋ/. It doesn't sound like "briev" to me here, unless they've changed the voice recording.