I'm wondering because of a composition I'm writing. I'm thinking of "Contrapunctus duplex ab octo vocibus" as a title for a movement.
No: if I have interpreted it correctly, "for eight voices" indicates the instrument by which the action takes place, which in this case are the voices of those who have to sing: the sentence must therefore be translated with per + accusative: “Contrapunctus duplex per octo voces”.
Moreover, it is a frequent formulation in musical composition: for an exempli gratia comparison, you can think of Sven-David Sandstrom's "Kyrie et Agnus Dei per tres voces" or of Johann Vinnubst's "Missa Dominicalis per tres voces cum organo".
EDIT: as it was shown in the comments and in the other answer, this kind of phrases have been translated in different ways over the years. However, the instrumental ablative could be fine if it is intended that the "voces" are musical instruments.
We have seen that also a / ab + ablative is very widespread in the traditional wording: this is because it presupposes a gerundive with a verbum cantandi (e.g. "Contrapunctus duplex ab octo vocibus canendus"); however the dative of agent could also work well ("Contrapunctus duplex octo vocibus canendus").
I am following up on Sebastian's comment. J.S. Bach gave Latin titles to many of his compositions, among them a “Canon triplex a 6 voc(ibus)”, and many others with the same construction. One could argue that this is a macaronic mixture of Latin with Italian or French. But one could also argue that it is wide-spread and standardised usage among composers of polyphonic music (which was, of course, unknown to the ancient Romans). I think if it was good enough for Bach it should be good enough for us, mere mortals that we are.
AN AFTERTHOUGHT: I have looked at this some more. The autograph superscription to the second part of the Matthew Passion reads: Pars 2da Passionis Christi secundum Matthaeum a due Chori per J.S. Bach. The first six words are Latin. “A due Chori” is not Latin, but unmistakably Italian, apart from the Latinising “h” (NB: Italian “a” is from ad, not ab). “Per”, to indicate the name of the author/composer, is neither Latin nor Italian; it seems rather to be a calque on French “par”.