The part of a fat volume that stores files and subdirectories is the:

The answer is Directory. It is an organizational unit within a storage medium that stores files, folders, and subdirectories. 
A directory is a location for storing files on your computer. They are found in the hierarchial file system, such as Linux, MS-DOS among others. 

The speed of  sound:   sound travels  at different speeds depending on what it istraveling through. of the three mediums (gas, liquid, and solid)  sound waves travel the slowest through  gases, faster  through  liquids, and fastest  through  solids.

Very hard to understand