Mr. Wole Soyinka, the Nigerian playwright and university lecturer, who has been standing trial at Ibadan on a charge of robbery with violence, was acquitted today. Mr. Justice Kayode Eso said there was a material conflict in the evidence of the prosecution witnesses. He was, however, convinced that the incident complained of in the charge really took place. Last October a gunman entered a Radio Nigeria studio in Ibadan, capital of western Nigeria, and attempted to replace the tape of a message from Chief Akintola, the Prime Minister, with his own. Mr. Soyinka said he was not in Ibadan at the time.