If you cast the Chipmunks and Chipettes who would you cast there are a few guidelines in casting
this is the voice before it gets pitched up
this is just the speaking voice (whether or not they can also sing is not important they could blend it with the singing voice)
the actor's/actress's voice has to match the character's personality
if you have sound editing software goof around with voices till you hear the chip if not maybe some computers have headphone settings where you can change the pitch (Don't know if works on speakers or tablets not sure not an IT worker)
and lastly you CAN NOT use the Bagdasarians
I know this may take awhile but give it a try
I will tell you mine in a soon if you're curious
GOOD LUCK