Earth connection missing on electric circuit, getting lots of noise with audio equipment
It shouldn't do any harm, and it doesn't even need to be copper, anything suitably conductive will do. I'd advise you to turn off the power before connecting it up just in case the existing system is mis-wired and leaking to earth. If it is, this will trip the RCD when you turn it back on.

Whether it will make any difference depends on the specific earthing arrangements of your kit and whether you have a ground loop among the instruments. Not all household appliances are connected to the earth pin at all.