It's a short little monograph, as the largest fact is that nobody knows much about the (language) culture of the Roman plebs because nobody wrote much down about it; the evidence he can cite is scene-setting, or circumstantial, or drawn from polemics against the plebs written centuries apart.
Horsfall's argument that the plebs probably had musical culture despite "their culpable unGreekness" p. 25) is a delicate outline; we know they sang, to memorize (arithmetic) or express political opinion or repeat the pleasures of the theater. Maybe we don't know what languages they could sing in. He thinks they probably knew some Greek, what with so many soldiers rotating through Greece, and so many Greek workers and slaves living in Rome. That would open the Greek plays the elites did write about to common enjoyment:
This is not to suggest that the mass public went to the theatre so as to learn Greek myth. There were indeed attractions of a very different order, but there is no profound incompatibility between unblushing delight taken in the most lurid special effects, flames, storms, battles, drives of animals, ghosts and a genuine love for the old tragedies.
That's page 59 and an argument for the serial comma.
The rest is detail, not all about music, pleasant if you like to imagine ancient Rome, not susceptible to more compression. He writes kindly of' accuracy in the details of material culture there.
Find in a LibrarySo wrote clew in History.