After helping Sun determine the father of her baby, Juliet secretly left a message on the recorder regarding the fertility of the pregnant women. ("D.O.C.") Ben later played the message in his tent at the ruins as Locke came to see him. Locke seemingly noted where Ben put the recorder (in a drawer) and later stole it and gave it to Sawyer, providing proof for his claim that Juliet is a spy at beach camp working for Ben. Sawyer took the tape recorder with him as he headed back to the beach. ("The Brig")
At the camp, Sawyer revealed the truth about Juliet by playing the tape. Juliet arrived and quickly instructed him to play the other side, where the survivors heard Ben's message that the Others planning to kidnap the fertile women. Juliet then revealed that she had already told Jack about the plan, and that they were still thinking of a counterplan. ("The Man Behind the Curtain")
However, it should be noted that this was the first time Juliet heard Ben's recorded message. At this point, Ben also had no way of knowing if she had received it or not.
JULIET: Ben, its six AM on Saturday morning. Kwon is pregnant, the fetus is healthy and was conceived on-Island with husband, he was sterile before they got here. I'm still working on getting samples from the other women, I should have Austen's soon. I'll report back when I know more.
[She turns off the recorder]
JULIET: [Pauses] I hate you.
Juliet, it's Ben. I'm sending three teams to extract Kwon the night after tomorrow. We won't have time to run Austen's sample so if you determine that she or anyone else is pregnant, mark their tents and we'll take them too. Good luck.