I have a feeling that if Sans knew the 'What does the fox say!" Song that he would get a video camera and say what does the fox say. Fox!Papy is just does that cute sound that foxes do. (Also I adore your blog and Stay awesome!)
Believe me, they chase Papy around with a camera for weeks when he made a noise for the first time. But the fox rarely do it, even BB hadn’t heard him much.
And yes, Sans knew about the song that time thank to Frisk.