One more question - Latin to English

I think vocibus verbi means something like "because of the voices
of the verbs". Immutari appears to have pronomina as its subject,
while varietate is the object of the preposition pro.

This would give you "Personal pronouns that are placed in front because of
the voices of the verbs [are] to be altered according to the difference
in the tenses and modes." And, yeah, it looks sketchy to me, too.

that is correct.