We know what is the true reality because of what you just said your self: The brain makes assumptions. We are aware of this shortcut and the ignorance that arises out of it. Due to this, we can eliminate these shortcuts and see reality objectively. To use the example you brought up: we are mostly empty space, correct. When I look at someone I don't see that. But that's due to our shortcomings in our sense organs like eyes. That being said, the fact that you know that we're mostly empty space hurts your arguments for two reasons: first. you're acknowledging this as objective fact, thus proving objective reality; second, we have a means of finding out what this objective reality entails through instruments that extend and perfect our faulty sensing organs.

Sound and light are different btw. Light is made of photons, sound is made from fluids compressing at a certain frequency.

Additionally, the fact that many people perceive reality differently doesn't mean reality is actually what we perceive through our sense. Our brain takes shortcuts all the time which is why we see faces in trees and animals in clouds. But we know objectively that not only our brain is taking shortcuts, but that reality is different than that misinterpretation. We can do this by, as stated above, instruments that extend and perfect our senses.

To emphasize further, reality is not shaped by perception. Sure quantum physics acts a little weird, but we still have more research to do there. That being said, if a tree falls and no one is around, it still makes a sound. It's quite a hubris statement to say that reality is defined by our perception. With that cleared up, multiple perspectives doesn't mean that there are more than one realities.

Finally, we know that color blind people or others who see music and feel light are experiencing reality differently, but again, we know what is objectively happening: Their brains are taking in objective physical phenomena like light and sound waves in different means than normal. But the physical reality of the objects remains objective and thus, reality as well.