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Can you speak while in gaseous form? The spell states that you can't cast spells with verbal components, but does not state why. If you can speak why can't you cast spells with verbal components?
IMO, You cannot. You're a cloud of gas. You're a sentient cloud of gas, yes, but you are gas nevertheless. I wouldn't let a gaseous character speak unless I planned on running a Ren & Stimpy campaign...
The writers probably decided that something like that was leaning mostly to the side of common sense. After all, if you're made of gas, and completely insubstantial, an indication that you can't cast spells with verbal components (which is really the only combat action requiring speech) would imply very strongly that speech is impossible while gaseous.