Apparently it's 6 months old [1]. You might be using the standard voice mode (the advanced one has just 1 voice IIUC).
[1] https://www.reddit.com/r/OpenAI/comments/1hdamrm/so_advanced...
[1] https://www.reddit.com/r/OpenAI/comments/1hdamrm/so_advanced...
Advanced https://help.openai.com/en/articles/9617425-advanced-voice-m...
Standard https://help.openai.com/en/articles/8400625-voice-mode-faq