This has been bothering me!!

That's what I was wondering. Then again, there are about 363,872.89 plot holes in this movie, and this is just one of them. Don't give it too much thought, it'll just make your brain hurt.

That's probably the least of the film's problems.

It does grind me how Megan Fox actually asks that same question, yet we're not given a clear answer.

Seize the moment, 'cause tomorrow you might be dead.

Well, if you consider tie in material to be canon Bee got his voice box damaged during a training exercise in a prequel comic by Starscream I think and was sent to stay with the Witwicky family.

COOKIES AND MILK!-Ed

His voice is still bothering him and he's playing it up.

It's literally explained right away in his very first scene.