I would not necessarily think of Latinization as a one way process or was anywhere as a uniform development. Throughout the Roman period, southern Italy maintained strong ties to a largely Greek speaking eastern Mediterranean. So even if a majority of the region spoke Latin, there would be quite a bit of Greek remaining. During the period between the reconquest of Justinian and the invasion of the Normans, southern Italy maintained strong ties to an increasingly Hellenized Roman Empire. The Medieval Salento by Linda Safran is an interesting microhistory of the heel of the boot where Greek was spoken in pockets until the 70's.

There are still to this day pockets of Greek speakers in Calabria, the so-called "toe" of the Italian Peninsula. There is disagreement over the origin of this dialect of Greek. Some argue its origin lies in Ancient Greek Colonization, while the more convincing argument is that it descends from Byzantine Greek. Nevertheless, there are records of continuous Greek communities in the south of Italy, albeit in pockets.

I don't know if this answers your questions, as you mentioned Naples, but hopefully it helps.

Beneventan chant developed in the 7th and 8th century was still sung in both Latin and Greek. Only after the forced migration to the gregorian chant repertoire in the 11th century we see the liturgy moving away from Greek.

Moreover bilingual Greek Latin singing is still attested in Naples in the 10th century.

You can search for Chants de la Cathédrale de Benevento to hear how it sounded like.