For example AudioSource#setVolume(float) is a link that is provided in AudioSource in the documentation for getVolume() but the setters are not defined by that interface.
Instead, these are implemented in only the AudioNode.
Would it be possible to declare the setters in the interface?
Why would you want them there? The interface is for the audio system to read what the audio source does. If you need your own interface just make one…? Not every AudioSource will have a setVolume method, maybe its automatically determining its volume or allows it to be set in dB?
Edit: I think you again make the same mistake as in the other post, (mis)using library classes for your own purposes/coding logic.