What do you think evolved first - verbal communication or thoughts? Presumably we were able to think before we could speak, no? The words we have in our language are like pointers to internal concepts, and it seems to me that those internal concepts would have existed before language was a thing. The mouth-sounds as you put it are not the thoughts themselves, rather just labels for specific concepts. It might be possible and even convenient to think in mouth-sounds but it’s not necessary for logical thought.