The first track is crickets only but slowed way down. In the background you can hear regular cricket chirps which were dubbed. The second track is crickets plus an ad lib opera singer. Two people say their cats went a little crazy and one person thought they were hearing the Vienna Boys Choir. h/t holykaw site.
NPR has an interview with the Sioux opera singer that includes her voice improvising together with the crickets beginning about 2:15 in the recording. Click on "Listen".
From Bonnie Joe Hunt's interview: "I kept thinking, ‘Oh, I almost can understand them. It’s a nice, mellow tone. And they never went off pitch until one of the interludes, where they went real crazy and they got back on again to where they were". And from the comments, two people say their cats went a little crazy and at another site, one person thought they were hearing the Vienna Boys Choir.
And one man questions the veracity. Is this reproducible? Good question.
UPDATE: I'm going to call bullshit on myself. My suspicions peaked as I listened to the bass section join the choir.