I have an older house, built in 1948. I upgraded the outlets after an electrician friend pointed out most of cable runs were actually already grounded, to the outlet box, not to the outlet; ie the cable was in a metal housing, and that was secured to the outlet box. All I had to do to ground the outlet was run a short wire from the ground screw on the outlet (usually a green screw, on the ones I bought at Home Depot) to the screw that held the outlet in. Which extended the ground from the outlet box to the outlet.
Of course you need access to the circuit breakers, and to map out your circuits first, for which that tool Dusty Chalk pictured is really useful. I don't know if the metal housed-cabling was common practice or not. There's a word for it I don't remember. I've never had an issue with a grounding hum, but I did have some random noise coming through my desktop amp and out some speakers; that I think had something to do with the computer power supply though, because grounding experiments didn't affect it, but running the amp off a battery fixed it.
