Odrade asks the worm a question, and the line is "there was no answer, but then she wasn't expecting an answer". Odrade had no ability to speak to the worms as Sheeana does, so on one level the meaning is obvious: "there was no answer because there couldn't be one". But to me it sounds like Herbert was going for something more profound. Did I misinterpret? Perhaps it means that Leto II was actually dead and Sheeana was either lying or misinterpreting her own control over the worms?
When the worm was joined with Leto II - the God Emperor, the Bene-Gesserit always wanted in on Leto's plan:
Our address to him will continue to be: "Tell us if we threaten you that we may desist." And: "Tell us of your grand plan that we may help." He has provided no new answers to either question during this period.
Page 53 God Emperor of Dune
Betraying his continued unwillingness throughout his reign to answer such questions. Why would Odrade expect any change in that pattern now?
Yes, the worms are mute as-such, and always symbolized the deepest animal-part of the God-Emperor's being when he was the worm, expressing itself only through sudden acts of violence. Even though "A Pearl of Awareness" remains within them from their union with Leto II, they still cannot directly communicate in concrete terms.
Leto II himself, on the first page of the first volume of his journals stated:
Questions are my enemies. For my questions explode! Answers leap up like a frightened flock, blackening the sky of my inescapable memories. Not one answer, not one suffices.
Which Darwi Odrade as Mother Superior of the Bene-Gesserit order would have been quite familiar with.
The preamble to Darwi's question explains the worm's language further:
"We have your language now," she said. There were no words in the language, only a moving, dancing adaptation to a moving, dancing universe. You could only speak the language, not translate it. To know the meaning you had to go through the experience and even then the meaning changed before your eyes. "Noble purpose" was, after all, an untranslatable experience. But when she looked down at the rough, heat-immune hide of that worm from the Rakian desert, Odrade knew what she saw: the visible evidence of noble purpose. Softly, she called down to him: "Hey! Old worm! Was this your design?"
It seems that the worm could only use this "untranslatable language" to communicate, so even if speech were possible, the meaning of any answer would be fluid and couldn't be pinned down.