It is audible in British English whereas it is mostly silent in American.

with dilemmas like this I really like to consult this website: http://www.howjsay.com/index.php?word=often It helps a lot, I couldn't learn how to pronounce "year" in ages, then I listened to it on this site over and over again until I managed to break my tongue into obeying. :) Just place your cursor over the word and voice will pronounce both versions, with and without 't'

Honestly? Your choice really. It isn't "a British thing" nor "an American thing". Personally, I don't pronounce the "t", but there are many fluent and good speakers who do. Either is correct.

No, it is silent. Mostly.

In my dialect, it's pronounced like a d. As in "ofden". In some dialects, it's pronounced like a normal t. "often" In yet other dialects, it's silent.