Sophie is just dumb. That's it. That's the whole running gag about her. She has an annoying, airy voice and talks kinda slowly.  She also extends her vowels out. So a simple "What?" turns into a "Whaaaat?" Also some of the words in her lines are misspelled on propose, so read them phonetically.  Also if the script ever says "cry, or scream" just go crazy. Personify this: KJABUIEFBIUQBIFUQBWOBNLWFNBOQWFOUIFN. Ya know? Too much, is not enough.  

There are sometimes where she's lucid, but just for comedic effect,or for when she's performing a spell. Oh yeah, she's kind of a witch now too, she just doesn't understand that concept though.

PLEASE! Listen to UpShutpls' audition below to get a good idea of what type of voice I want.