There are a few different tunes:

I

gregobase and youtube thanks to Neumes and Tunes

II

This is the first one I learned. gregobase and youtube

III

gregobase and youtube thanks to Cathaholic Music

IV

Floriani recorded this one:

gregobase for sheet music, youtube for video.

V

gregobase and youtube thanks to Verbum Gloriae

Marco Frisina

and a polyphonic one by Marco Frisina which is still under copyright, so you’ll need to buy the sheet music and it is so ubiquitous, I’m sure you can find recordings.