I think I'll hazard a guess too, even though I haven't figured the title out yet, since the fit on the final three clues seems more or less airtight (heh).
Are you, by any chance, a
I associate with a group and take only two
Koffing is a Pokémon with only two evolutions, Koffing and Weezing.
I am a sound most people hear in their lives
Most people hear coughing in their lives
Root minus dairy plus letter mixed around
..Yeah, haven't figured out this part. Sorry.
A large boulder rests upon my shoulders
Wrapped around a man I lay
That's exactly how you use a coffin.
Flipped open in the beginning of the birth of a new being
This clue seems to be carefully avoiding the words "life" and "human", so..
A vampire's existence begins with an opening coffin.