GPT-4o voice isn't out yet, so you were likely chatting with the old/current tech (which is still really good).
From OpenAI: "We'll roll out a new version of Voice Mode with GPT-4o in alpha within ChatGPT Plus in the coming weeks"
From OpenAI: "We'll roll out a new version of Voice Mode with GPT-4o in alpha within ChatGPT Plus in the coming weeks"