Commentary Purg II 75

This final detail, on the other hand, does inject a note of moral tension, since it reveals the predisposition of these souls to linger where they are rather than to proceed upward.  See C.Purg.II.10-12.  Their openness to negligence begins the conflict narrated in this second section of the canto, in which Casella's song will so engage them that they will finally have to be chastised by Cato.  See Francesco da Buti's comment to this verse (DDP Buti.Purg.II.75), insisting on 'their negligence, which follows from their delight in worldly things.'

      The verse carries an overtone of a line in the passage from Convivio describing the effect of Mars as music, moving the hearers to such a state that they 'quasi cessano da ogni operazione' (almost completely cease their activity).  See C.Purg.II.13-18.  Dante is, in effect, their 'music,' and they respond more to the miracle of his fleshly presence than to the sign of God's love that it gives, and which should be acknowledged.