One easy way to Get the Building Grounded is to Report it to the State Building Inspector and he will force the Owner to Ground the Buildings Power as Non grounded Outlets can be Dangerous and are against the Building Codes of most Civilized Countries.....
Another way if you want to do it yourself is To hang a Wire with a Nail on the End off of your Balcony and Hammer the Nail into the Ground and connect the Other end of the Wire to the Ground Plug on the Power Bar you use for your audio equipment which should Ground the Power bar...It sounds Very Mickey mouse but is an easy way to do it......
It"s Gotta suck trying to record when everything has that Bad Humm in the Backround!!
Cheers