The former is the most common way for Americans to pronounce it, and matches the mouth shape of ah, and the latter, while not common in the US, also matches the ah mouth shape. I really don't know how yesenadam is pronouncing it such that his mouth doesn't take the ah shape.
Ohhh. I'm sorry everyone. Yes, you're right of course. It didn't occur to me until I tried it that the bu- in bun is bahhh. Goes back to drawing board