Voder boxes were commonly used by races and cultures who could not produce speech as most other humanoids perceive it. For example, Starfleet ensign Naraht, a Horta, wore a voder box that converted his attempts to communicate with the outside world into an artificial voice that could speak Federation Standard speech. Naraht's voder was also marked with a Starfleet insignia badge and signified rank, in the 2270s as a series of rank insignia stripes (see Federation Starfleet ranks (2270s)). (TOS - Rihannsu novels: My Enemy, My Ally, The Romulan Way)
A vocal encoder was part of a universal translator, whether it be in a starship's communication system or in a portable translator wand. The UT technology was advanced enough to process concepts like age and gender in non-humanoid aliens, and assign them voices appropriate to that input. A translator wand carried by Spock on Gamma Canaris 527 was able to assign a vital female voice to the Companion on the basis of it's perceived female nature. (TOS episode & Star Trek 7 novelization: Metamorphosis)
Some advanced forms of vocal encoding tie directly in to a humanoid auditory and nervous systems, allowing a being's own voice to be heard in the language being translated. Intradermal translators of this type are delicate instruments, and usually rely on updates from a larger computer system, meaning they may fail in the absence of computer contact or in turbulent situations. These translators must also carry programming for a limited number of languages, meaning that unexpected translations (into languages not published into the translator memory) might fail. A device made with a cesium-rubidium crystal injected under a brachial nerve would expedite this process.
Quark, Rom and Nog all routinely wore translators in their ear canals for everyday translation, as they could not understand a variety of languages spoken on Deep Space 9. After an energy blast propelled them back in time, the translators failed, leaving them unable to understand English. When in full repair, their translators used the Ferengi's own voices as vocoders, emitting speech in the desired language, without the past Humans being able to perceive that the speech was being changed by the device. (DS9 episodes: "Sanctuary", "Little Green Men"; TOS - Rihannsu novels: My Enemy, My Ally, The Romulan Way; TOS novel: Rules of Engagement)
A vocoder could be assembled as a prosthesis to attach to a lifeform that has lost vocal capability. A device of this type was fitten to the Bajoran terrorist Orta after his vocal cords were cut by a Cardassian, allowing him to speak in a mechanical simulacrum of his own voice. (TNG episode: "Ensign Ro")