My first is in donut and also in dough
My second in grew, but never in grow
My third is in famine but never in meat
My fourth is in sofa but never in seat
My fifth is in soldier, but never in sarge
My sixth is in flabby and also in large
My seventh in twentieth, not twenty-third
My whole helps with hearing before sounds are heard.
Hint:
There isn't a mistake with my seventh.