It is possible that whoever installed the outlets didn’t connect a ground to the box, although just connecting it should have grounded it. There will either be conduit or BX running through the walls.
Do you have access to the fuse box? Are they breakers or screw in fuses?
I have to agree with Brodric, you should have an electrician out, both to inspect all of the wiring, and have him install at least three dedicated circuits. If you decide to buy a P20 you will need a 20 amp circuit with a 20 amp outlet. I would have one 15 and two 20 amp at a minimum. With at least one of the 20s back behind the speakers. For a sub or two, and if you go to monoblocks.
If you deal with a local audio dealer, ask him to recommend an electrician who understands the needs of an audio or videophile. One that will install heavier gauge wire than code requires.