Gram loquitur, Dia ver docet, Rhe verba colorat,and searching for that gave me this Seven Liberal Arts post from the Common Room. It is part of a whole series and the comments on the article are interesting.
Mus canit, ar numerat Geo ponderat As colit astra.
Christine Miller at Classical Christian Homeschooling says that CS Lewis quoted the couplet in The Discarded Image, and translated it this way:
Grammar talks, Dialectic teaches words, Rhetoric colors words,
Music sings, Arithmetic numbers, Geometry weighs, Astronomy tends the stars.
ETA: Drew pointed out that the Latin from the AO article didn't match the English translation (see comments). Apparently CS Lewis remembered the tag as "Dia verba docet" and translated it accordingly. I didn't notice the difference between the two versions when I pasted them in.
Other sources, like the Catholic Encyclopedia entry on Liberal Arts, had it as
Dia vera docet
The whole mnemonic there is:
Lingua, tropus, ratio, numerus, tonus, angulus, astra.
Gram. loquiter, Dia. vera docet, Rhe. verba colorat
Mu. canit, Ar. numerat, Geo. ponderat, Ast. colit astra.
That's different again. (By the way, that's a typo I found above in the CE. It ought to be "loquitur", and it is written that way in the original CE text. )
I need a librarian emoticon ;-).