"What we've got here is a case where a drug deal is going down and there are cell phones used that are encrypted with tones," St. Croix said. "So that if we intercept, all we hear is the screaming tones."
But with some technical intervention, which included removing the screaming tones, voices were audible on the same tape. With the voices made clear, police have evidence that may be used in court.
The National Archives, desperate to fill in the legendary gap before the Nixon tapes disintegrate, is auditioning experts, including St. Croix, for the job.
Candidates got an erased test tape to show off their skills. If any of the techs are given the original Nixon tape, the National Archives wants to make sure that they will not damage the tape, as it is a historical artifact.
In the meantime, experts including St. Croix believe they can get Nixon's voice back.
"There's always a holy grail," St. Croix said. "So in forensic audio historical recovery, this is it."
Historians wonder what on earth Nixon was trying to hide, since all the other tapes were already so damning, with blatant evidence of a cover-up.
"I have no idea, but it must have been extremely embarrassing," said Ben Bradlee, who was editor at The Washington Post when reporters Bob Woodward and Carl Bernstein broke the story. He is now Vice President at Large at the paper.
Then, there's Deep Throat, the shadowy source so dramatically portrayed in All the President's Men, a movie about how Woodward and Bernstein's investigation into the Watergate scandal set the stage for Nixon's eventual resignation.
Today, other than Deep Throat himself, only three people — Woodward, Bernstein and Bradlee — know the identity of Deep Throat. Former Nixon White House counsel John Dean claims to know, and was set to publish his latest theory today on Salon.com. But his publisher is scrambling to re-work Dean's online book, after his suspected Deep Throat denied it.
Just working in the Nixon White House made people candidates for being the secret source. ABCNEWS' own Diane Sawyer, who held several positions in the Nixon administration, was on the list, until the reporters made it clear that Deep Throat is a man.
Woodward and Bernstein have said the mystery source is not Alexander Haig, who was Deputy National Security Advisor during Watergate. But through the years, the reporters have steadfastly declined to reveal who Deep Throat is when names have been put before them.
"You know none of the three of us, Bob myself or Ben Bradlee, have gotten into the guessing game," Bernstein said.
But plenty of others have. Dean has now named three different Deep Throats, and all have denied being the one.
Bradlee says that eventually, that one day, we will all know this Watergate secret. "Sure you're going to," Bradlee said. "Woodward has said that they would reveal Deep Throat's identity when he dies."
Ultimately, the most profound Watergate puzzle may be Nixon himself. He was handily set to win re-election, yet so plagued by insecurity that he became a criminal.
"The thing that baffles me most of all is what was in Richard Nixon's brain," presidential historian Michael Beschloss said.
"Here was a man who was totally unsuited for the presidency of the United States," Bernstein added. "I think that's where the great mystery lies. How did this person get to be who he was?"