A jury of five men and seven women was selected yesterday in U.S. District Court in the retrail of two anti-Castro Cubans charged with the 1976 car-bombing assassination of former Chilean ambassador Orlando Letelier and an associate, Ronni Karpen Moffitt.

Opening statements in the case were delayed yesterday while prosecution and defense lawyers argued over the use at the retrail of transcripts of a taped telephone conversation between the government's key witness, Michael Vernon Townley, and a friend in Santiago, Chile, which was disclosed during the last week of the orginial 1979 trial.

When the U.S. Court of Appeals reversed the convictions of the two Cubans last September, it also ruled that presiding Judge Barrington D. Parker erred when he refused to allow defense lawyers to cross-examine Townley about the conversation. According to defense transcripts of the telephone call, Townley made disparaging remarks about Parker and the jury, and said he would have friends threaten Parker so that he would withdraw from the case.

Yesterday, Townley took the witness stand, outside the presence of the jury, and testified that portions of the conversation are omitted in the transcripts. The tape of the conversation was made available to defense lawyers by an attorney for the head of the Chilean secret police, known as DINA. The origin of the tape and how it was made are unknown, although the tape is believed to have been made in Chile.

The two defendants, Guillermo Novo Sampol and Alvin Ross Diaz, are accused of conspiring with six others to carry out the murder of Letelier, who was killed in a midmorning explosion at Sheridan Circle on Sept. 21, 1976. Townley, an American-born DINA agent, testified at the original trail that he recruited the Cubans to help him carry out the plot at the direction of his DinaY superiors.

The Chilean courts have refused to extradite the secrete police officials and two operatives indicted in the Letelier case. Two other indicted Cubans are fugitives, and a third is waiting a retrial.

Defense lawyersd Paul Goldberger and Lawrence Dubin contend that the transcripts of the conversations are "crucial" to their efforts to show Townley is biased against the court. They also argue that the conversation undercuts his truthfulness as a witness because his comments show a contempt for the judicial system.

Federal prosecutors, however, have challenged the authenticity of the tape because of the mystery surrounding its origin and contend that the prejudicial impact of Townley's statements on the jury would outweigh any value they might have in challenging his credibility.

Parker said yesterday that he would not allow defense lawyers to question Townley on any statements he made about the court or the jury, which Parker said were "inflammatory" as well as irrelevant to the case.

The hearing on the transcripts is expected to resume today.