It’s something about that particular sound, not necessarily that voice, as one mom shows that simply singing it to her kid herself elicits a similar response.
This next video ends prior to the kid freaking out, but it’s perfectly clear what’s about to happen.
It seems that the voice of AURORA, who provides the call that only Elsa hears, is causing small child breakdows.
And it turns out that, yes, this is something other parents are dealing with.