On macOS, the returned list contains the built-in operating system voices (about 50) and some additional ones by Google (roughly 20). The spec [1] says the voic

On macOS, the returned list contains the built-in operating system voices (about 50) and some additional ones by Google (roughly 20). The spec [1] says the voices made available are entirely up to the browser. Doesn't really answer your question, but at least some additional insight. [1] https://w3c.github.io/speech-api/#dom-speechsynthesis-getvoices https://w3c.github.io/speech-api/#dom-speechsynthesis-getvoi...