Air NZ did this about 2 and 1/2 years ago on a A320 AKL-SYD for the Australian MTV Music Video Awards, and again in 2008.
It was sold as a commercial flight and was a success as I recall, wasn't a drop of booze left on the plane when it got into SYD.
Speakers were placed under the seats of roughly every 6-7 rows, with a couple of rows fully occupied by amps, band equipment etc...
Lots of fat cables running through the cabin, all tied together leading to a big transformer strapped into the fwd galley where a oven/can would go.
Power supply was directly linked to the cockpit 'GALLEY' switch on the overhead panel incase things got a wee bit hot...
2007
YouTube - Mile High Gig
2008
YouTube - MTV Mile High gig 2008 on 3news