Grounding

The metal rail is a bad place being on an upper floor of a balcony. I would run it to cold water pipe instead hoping it is an all metal pipe going into the ground

Concrete is poor conductor, especially when it is dry.

Normally the ground on the AC outlet is fine.

If your system needs more than that, take it as an indication that the ground circuits in one or more components in the system are not set up ideally for best results.

cold water pipe in the place i live is plastic and so sewage. one way is to drop ground inside sewage such as drop it in toilet tub, but be careful not to do ya thing when there's thunder or you can loose ya things...
sewage and waste is much better conductor vs. water.

First, I'm not sure what it is that you are referring to that you want to connect to an external earth ground point. Are you talking about grounding the chassis of the equipment, which would normally be grounded anyway via the AC safety ground pin of the power plugs (assuming they are 3-prong plugs)? Or are you talking about connecting the signal grounds/circuit grounds of the system to earth ground?

In either case, though, I agree with Ralph/Atmasphere that if your equipment is properly designed you are unlikely to be accomplishing anything constructive by adding extraneous ground connections. In addition, you would probably be introducing a code violation, and possibly serious safety hazards as well. See the upper part of page 8 of this paper.

Regards,
-- Al