Ask HN: How to sync background music to voice?

I am working on a side project where I am able to combine the voice from one music video to the background music of another video.

i.e Taylor Swift's voice from Last Christmas to the BGM of Van Halen's jump etc.

It is just not working. Are there any techniques to programmatically add the bgm appropriately?

Right now, I am thinking of reducing the BGM audio while the vocals are being played.

What is the right direction I need to take?