In chapter 7 of Things Fall Apart, indeed a silence falls upon the Ibo clansmen, for they know they must commit the unspeakable: kill Ikemefuna. Achebe describes the event as a kind of apocolypse:
the locusts came...a shadow fell on the world...the sun seemed hidden behind a thick cloud..."Locusts are descending."
After the locusts, Okonkwo gets words from Ezeudu that the village has decided to kill Ikemefuna: "the Oracle of the Hills and Caves has pronounced it." Ezeudu warns Okonkwo not to take part.
So, the entire event is shrouded in silence: we never hear why the villagers want Ikemefuna killed; we never hear the oracle (can one hear such a thing?); and Okonkwo never tells his son Nwoye or his adopted son Ikemefuna. The murder is shrouded in silence to protect the village.
We may possibly assume Umuofia wants the boy dead because he is reaching manhood, because he is stronger than Nwoye and therefore a threat to eventually taking a leadership role in the clan, and because he is an outsider. So, the communal village must protect itself against outside threats.
On the day of the announcement there is an eerie silence on the land:
The harmattan was in the air and seemed to distill a hazy feeling of sleep on the world. Okonkwo and the boys worked in complete silence...
On the day of the murder, more silence:
Okonkwo got ready quickly and the party set out with Ikemefuna carrying the pot of wine. A deathly silence descended on Okonkwo's compound. Even the very little children seemed to know. Throughout that day Nwoye sat in his mother's hut and tears stood in his eyes...
And, as the men depart:
At the beginning of their journey the men of Umuofia talked and laughed about the locusts, about their women, and about some effeminate men who had refused to come with them. But as they drew near to the outskirts of Umuofia silence fell upon them too...
They argued for a short while and fell into silence again, and the elusive dance rose and fell with the wind. Somewhere a man was taking one of the titles of his clan, with music and dancing and a great feast.
So, everyone knows but cannot speak: the men, the women, the effeminate men, the children, and especially Nwoye. He is angry beyond words. Okonwko too is angry, but he dare not speak, lest Ikemfuna cry out.
When the time of the murder comes, Okonkwo lashes out and kills the boy, despite orders to the contrary, with nary a word. In fact, he kills the boy because he broke the silence, saying "My father they have killed me!" So, to Okonkwo, words are weak. They remind him of his lazy agbala father. Better to fight to the sounds of the drum, better to work under the hot sun, better to wrestle in the ring. Words are weak. Silence is manly.