A ground cable is used for safety. It does not affect the sound unless your system is miswired.
It should be rated at the equivalent or greater current level which would blow the main fuse of your specific device. To be sure, let us say at twice the level. For most purposes and for devices connected to a 15 amp mains, it is not necessary to have a ground cable thicker than about 10 AWG, unless you somehow intend to bypass your fuse (I know there are some fuse lovers on this thread) and run your device as a blast furnace.
Paying any thing over a few dollars for it is just plain .........