“For rupa Brahma’s since they are missing the faculties of “touch” and “taste”. Without taste, that means they don’t have a mouth. How do they communicate with Lord Buddha?”
Yes. The answers by y not and Sharma27 are close.
The succinct way to say that is the following.
It is the sota pasada rupa that makes it possible to communicate with others.
– Rupavcara Brahmas (as well as human gandhabbas) communicate directly (without words) via that sota pasada rupa.
– It is only when the gandhabba is trapped in a physical body that mouths and ears (and air vibrations for sound propagation) are needed to communicate.
– Arupavacara Brahmas (and those in the asanna realm) do not have the sota pasada rupa. That is why they cannot communicate with others.