In other words, [screen reader vendors] decided to protect their users from authors because authors seemed not to care about accessibility. Now imagine what would happen if such authors were able to change the aural presentation of a document through CSS. For example, they might specify a background sound played aloud on every single page, thus disturbing the normal reading of the program, or lower the volume of certain elements under a reasonable level and so on. To put it simple: CSS aural is not supported because it poses more accessibility problems than it can actually solve.
Why aural CSS is not supported by screen readers via @sprungmarkers

Source: onwebdev.blogspot.com