I think I can see the poblem in you list of sinks:

 ```bash
   * index: 0
     name: 
  ```

 the * indicates that this is selected as default. 
 So cava will try to grab the audio being played to this output and 
 not the Bluetooth headphone.

 Try to go to your system sound settings and select the Bluetooth headphones 
 in the output settings there. *Before* launching cava.