A vocabulator, also known as a vocoder, was a speech synthesizer that was fitted to almost all types of droids that were intended to interact with sentient beings. Some droids, such as HK-series assassin droids and protocol droids—which were commonly fluent in many galactic languages—could use their vocabulators to mimic or impersonate the voices of other beings or creatures.
The Tranlang III Vocabulator was one such model that was used in the 3PO-series protocol droid, granting models of this line the ability to articulate in over six million forms of communication. This model of vocoder also saw use in governmental droids.
Droids lacking a vocabulator could resort to external translator units.
Vocoders could also be fitted as cyborg attachments for organic beings that had suffered injuries that cost them their ability to speak. Darth Malak was equipped with a vocoder attachment that served to replace his severed jaw, as was Darth Vader almost 4,000 years later. In Vader's case, the vocoder served to enhance what little speaking ability he had left after his vocal cords were scorched on Mustafar. Wat Tambor also used such a device, although he was shown to have to frequently adjust it using attachments on his metal suit as one would tune a radio.
A transliterator was a similar device. It translated speech into another programmed voice.
Behind the scenes
There is a real-world electronic device called a vocoder with communication and musical uses. This bears no resemblance to the Star Wars device.
Galaxy Guide 3: The Empire Strikes Back spells the device "vocodor."
In theory, vocabulators appear in any work that has a speaking droid. The list below includes only works specifically mentioning a vocabulator or vocoder.