Hi Rainer,
I can't tell when that audio clip comes from in the mission - it's what you often hear when the crew are speaking among themselves - but aren't speaking to the ground. Enough of their voice breaks through that you can hear them (but not tell what they are saying) if the background is quiet enough.
I realised this was happening when comparing onboard audio (recorded with the DSE) with what was heard on the ground.
Though I haven't put together a comparison file, you can heard the same effect on Apollo 8 - just find a spot in the mission where there is a good recording of the onboard audio and also a recording of the downlinked voice. On this page -
<https://honeysucklecreek.net/msfn_missions/Apollo_8_mission/apollo8_audio_highlights.html>
I think you'll find some overlaps.
I guess the producers just put it in there to help it sound authentic. :-)
Colin