Read pc volume percentage?
There is no way for any hardware device, Arduino or otherwise, to request information like that from a computer. 

HID is a protocol which (when used as a keyboard) allows you to send keypresses, including "media keys", which tell the computer to change the volume (amongst other things). There is no provision, other than three status LEDs, for getting any data back from the computer.

You will have to write software for your computer to query that information using whatever API may be available for your operating system, and pass that information through whatever communication channel you choose to the Arduino (such as through serial).

Some pointers: 


C# get master volume level/precent
Windows 7 Master Volume
Arduino: Sending and Receiving Multi-Digit Integers