If I'm not wrong you can actually see someone sitting on a couch behind the huge grey sticker on a video. And it seems that they might be holding Altas. So most likely you are right and it is Atlas coos.Those last two clips are Atlas' "coos" - pay attention closely. I can't tell if they're pre-recorded or if he's just outside the camera range, but she's attributing actual baby coo noises to Luna and it's blatant lying fuckery. She's a lying liar who lies, so this is not shocking.
Puting a sticker over a functioning baby to make Luna sound "normal"