iMac 21.5" interference noise in external speakers

I have a brand new iMac 21.5", that I use also for music production. When the computer is connected to external speakers (M-Audio) through audio out, there is a strange constant noise - interference (?). Plus additional noise accompanying every move of the wireless mouse or scrolling, typing on the keyboard, hiding and showing the Dock, even pictures changing in the browser (banners) without any hardware activity. When the screen goes darker (saving mode) the noise becomes unbearable.

I just upgraded from the 2007 iMac with USB mouse and keyboard and in the exactly the same setup I hadn't experienced any problems. As well with my iPod - no problems at all.

I tried with various audio cables without any difference.
Would this be caused by Bluetooth? Grounding? Shielding? (Speakers are shielded.)

I called yesterday the Apple Support and after trying through the phone I was advised to go to the Genius Bar at the local Apple store. As they couldn't check the cause in the Genius Bar (didn't have speakers available ...), I got the iMac exchanged for a brand new piece. But now - back at home, the noise is still there.

Please help!