The only decent one that I’ve found is this:
Here’s an example usage:
Depending on the format you want it to return it’ll return the title, link to direct mp3 download and length. Unfortunately it has a daily limit ( I don’t know what number exactly, I just know that after a while it stopped working, when I checked the website it said I had reached my daily limit ) It also appears that videos that hadn’t been previously converted, need to be converted and it will take multiple tries until you get a working download link.
If you wish to make your own, you’re going to need to host a conversion service.