Some PTZs do have Mic and Speakers built in, But seems like yours may not.. So would as said before need an external Mic. Then even when you enable from camera at times it seems there might be an issue even with audio after being enable using WebUI. I found in this case you need to go to your cameras App and click on the camera, Then the Device info and at times there is an enable Audio button that needs to be turned on by the app.. It wasn't like this before for me. It wasn't until the last app update that I had to start doing this,
So if you happen to have an external mic installed, enabled it by WebUI and making sure that it is setup for External some cameras even without mic found they have both options and need to change from Mic to External, Again even in live play back aka Live View there is no audio then use the app and enable it in the app..
App camera, icon top right, more, and picture below shows area for reference