There's plenty of performances of this medieval tune available, but I've picked this one out in particular because it's a good opportunity to see less familiar instruments. It also clearly demonstrates the use of drones, which fits in well with the arrangement on the ABRSM Grade 3 syllabus.
The conductor Michael Tilson Thomas offers the best explanation of the unique power of classical music I've come across. He includes a whirlwind trip through the history of notation and of the evolution of harmony, but also enthuses about the almost mystical connection music can have with us, and how important it is for us to continue that transmission.
By special request, a selection of performances of this piece, in various versions.
First, Brahms' own orchestral version, with the Berlin Philharmonic and Claudio Abbado:
Good videos of these pieces are proving rather difficult to find, but here's a couple.
First, a chance to see more of this warm, endearing performance of Mozart by Gil Shaham (the first movement also a grade 7 piece, with ABRSM):