So, being super into undertale, i've found some frankly amazing fan music videos with undertale animations.

The first three are parodies of the "stronger than you" song, sans, and then frisk and chara's responses.

The next is an animation with a piano/vocals cover of megalovania, along with a small bit of the "Dummy! Remix" song at the end, both fantastic songs.

and finally, waters of megalovania

I have a thing about sans, in case you haven't noticed. anyways, great fan music all around.