How about this... I am not sure about a whole Flaw about not speaking but I will give your Animal Form one "conditional" Feat. It will change with each form, mostly as a 'Skill Focus' or 'Talent'.
So Wolf, most of the big cats, etc might have +2 to Spot and Listen
A Panter or other sneaky type might get +2 to Move Silent and Hide.
Bears might get +3 to listen (as they don't have good sight.
Any bird or Hawk would get +3 to Sight
Rhino, etc might get ah... attack focus with its big horn, etc.
what do you think?