That is not what I meant, in the context of an observation Haki users ability to react to them, they have similar properties
Niether light nor sound are themselves not forms of matter. However, both light and sound either originate from matter or occur as a result of the interactions and movements of matter.
Hence if CoO can help you react to light, it can also help you react to sound. Vibrations doesn't hinder your ability to react to that, it only helps.
Or are you suggesting Apoo is a sole character in the entire show whose attacks are observation Haki proof, are you gonna grant Apoo that just so you can boost Luffy?