To me, that sounds like a baby is talking/singing on the baby monitor. That's not the device sounding, that's the child actually saying words. (And that alerts the people to let them know the baby is awake.)
* Most baby monitors I'm familiar with are basically just radios - the sounds the baby makes can be heard wherever the other monitor is. (It would surprise me if the machine was making that signal. I really do think it's the kid talking)
* Small children (ones that you need a baby monitor for), often make grammatical mistakes. Also, it isn't that important what the child is saying, it is more important that she woke up and someone needs to go care for her.