This is so beautifully written! I love the analogies that you draw between their relationship and music and sounds. The smooth sound of a symphony compared to the cacophonous noise of various cries.

Byakuya lived following the melodic sounds of his family's orchestra.

Renji survived through the passionate cries of the battlefields.

But when together, their hearts beat in synch to their own accord.

I loved these three lines. Wonderful job!