The sound of using the brush isn't affected by blocks

Confirmed.

Any action that is created through an entity doing something that results in blocks being changed, is normally controlled by the "Blocks" sound slider. With this example, the sound of brushing suspicious sand should indeed be controlled by the "Blocks" sound slider, as the suspicious sand changes its "dusted" block state when a player brushes it. On the other hand, as for brushing non-suspicious sand blocks, the blocks don't change in any way, but I think the sounds produced should still be bound to the "Blocks" sound slider since the blocks being brushed are technically being interacted with, despite them not changing in any way.

It seems to be affected by the "Players" sound slider.

The "Brushing Sand completed" or "Brushing Gravel completed" sounds were not fixed.