1) In the collection settings tab, check "Speech to text available for transcribing." Select the language of the transcription text:
2) Voice dictation only works with Google Chrome, so ensure transcribers are using that browser.
3) Each transcriber needs to set the language they will be speaking in their user profile. On the top right of the screen, click "Signed in as..." then choose "Your profile." Then click "Edit Profile." In the user profile look for "default dictation language" and choose the language, and then the locale, for the language you will be dictating in.
Once all three are in place you'll get a microphone icon in the "Transcribe" tab; click on it and speak into a microphone to use the voice dictation.