WASHINGTON — Prosecutors at the CIA leak trial yesterday started playing audiotapes of I. Lewis Libby’s grand jury testimony, a key piece of evidence against Vice President Cheney’s former chief of staff.

It was the first time that the trial jurors had heard Mr. Libby ‘s voice.

On the taped grand jury testimony, Special Counsel Patrick Fitzgerald began by asking Mr. Libby to spell his name and asking if he has a nickname.

“Scooter,” Mr. Libby replied.

Asked to explain, Mr. Libby joked, “Are we classified in here?” and then said that in the South, where he is from, such a nickname is “less uncommon than it is up here.”

Mr. Fitzgerald reminded Mr. Libby that a person who doesn’t tell the truth in front of a grand jury could be charged with perjury. Mr. Libby said he understood.

“I am” prepared to proceed, Mr. Libby said.

The tapes, which will be publicly released and broadcast once the trial jury finishes hearing them, form the basis for three of the five criminal charges against Mr. Libby, who is accused of perjury, obstruction, and lying to the FBI.

Mr. Libby is charged with lying about how he learned of the CIA identity of the wife of Bush administration critic Joseph Wilson and what he told reporters about it.

Portions of the obstruction charge as well as both perjury charges deal with the ex-White House aide’s alleged lies to the grand jury.

Mr. Libby’s alleged lies in his 2004 grand jury testimony concern his conversations with NBC News reporter Tim Russert, New York Times reporter Judith Miller, and Time magazine reporter Matthew Cooper.

Prosecutors plan to play a number of hours of Mr. Libby’s grand jury testimony for jurors. He made lengthy grand jury appearances on March 5 and March 24 of 2004.

It is unusual for the government to make such extensive use of a defendant’s audiotaped grand jury testimony. The federal government did so 17 years ago in another high-profile criminal case in Washington, D.C., the drug trial of a former mayor, Marion Barry.