Can anyone help me find royalties info on this

It is possible that a lot of groups have performed the music because it is in "public domain", meaning that either the copyright has expired, or it was never registered in the first place. Do some research on public domain mariachi music and see what you can find. Anything in public domain is free to use.
Maybe this is what you're describing that you've already done, but try doing a publisher search for the song information:

Put Las Alazanas in the search box and you'll see the publisher's information on the right hand side (towards the bottom). Click on the black arrow next to the name and all of their contact information will be visible. From there you can contact them and talk about agreements, costs, etc for using this piece in your film.

Might even want to be sure and send that youtube link as well just to make sure that it's the same song they're over.

Hope that helps a little.

- Mike
It's very possible that it's a public domain tune. However, that would mean that you'd still need 'master use' rights to use that actual recording (or any recording of it, for that matter). If the song is not public domain, then you would need both synchronization rights and master use rights.

Another option if it's not public domain is to just get synchronization rights, then have a group perform/record a cover of the tune for your project directly.

If it is public domain, as I suspect, I'm guessing you'd be able to find a reasonably priced cover of it from a lesser known group in order to get the master use rights of that recording. The specific one on that youtube page may lean toward more expensive than lesser known recordings.