Why are conscious visual experiences different from conscious auditory experiences?

I'm interested in finding anything written from a physicalist or physical-panpsychist perspective as to why--given that they both presumably arise from neural activity--conscious visual experiences are different in quality from conscious auditory experiences (this applies to any of the senses, of course).

Are different kinds of neurons involved? Different patterns of activity? If so, how would that give rise to different experiential qualities given that the same fundamental particles would still be involved?

Does synaesthesia shed any light on the question?

Anyway, if anyone can point me to any writing on the question, I'd be very appreciative. Thanks!