I am using the latest library version 4.6.1.
Some of the voices listed at https://platform.openai.com/docs/api-reference/audio/createSpeech#audio-createspeech-voice ("alloy", "ash", "ballad", "coral", "echo", "fable", "onyx", "nova", "sage", "shimmer" and "verse") do not have a corresponding constant in SpeechCreateParams.Voice (https://javadoc.io/doc/com.openai/openai-java/latest/com/openai/models/audio/speech/SpeechCreateParams.Voice.html).
In particular, "fable", "onyx" and "nova" are missing, and the Java library provides constants CEDAR and MARIN that don't appear to match up with supported voices in the speech synthesis API.