Is it possible to integrate a voice chat with openVidu?
Yes. You can create a voice only video chat.
Do you have any demo for the audio chat
Nop. But you can use any demo and disable video when creating Publisher passing a false in property videoSource in the publisherProperties param of OpenVidu.initPublisher(...).
https://docs.openvidu.io/en/2.13.0/api/openvidu-browser/interfaces/publisherproperties.html