Actually I think he wants to actually sustain power to the PC and monitor, letting them go into standby mode while at the same time triggering the lights and speakers to switch off.
Before the power strip mentioned became known, I went the "single button power the cab" route of wiring a relay switch into a power strip which was in turn triggered by 12v running off the PC, power the PC and then it would trigger power to the power strip.
I think for what you describe you would need some type of similar relay that is going to trigger "off" when the PC hits standby mode....I've never tested it but do the 12v and 5v lines go low when PC hits standby mode? If yes, that would work.
Or perhaps the USB ports?
Again, I think wanting to cut power to certain devices will require the wiring of some sort of relay that is triggered by the fact that the PC went into standby mode.
Hope this helps....