Arcam AVR200. Buzzing sounds in all 5.1 speakers. Gets louder as volume is turned up. Tried all sorts of different things - different power source, 1 speaker, different modes.

I had the same problem. I was told by a sales guy at a high end audio store that I should try using a 3-Prong To 2-Prong Adapter. I went and bought one. I plugged it in and no more buzzing.

you can find them at HomeDepot, Lowes or dollor store.

If it gets louder with the volume it's noise entering in or before the preamp area.

There could be an unsecured cable shield/ground or a major ground potential difference between the Arcam and other equipment. Try floating the ARCAM free of any other external electronics first. If it's silent start adding things until you identify the culprit.

Other advice:

http://www.turtlebeach.com/support/index.php?View=entry&EntryID=830516298

... which mentions Cable TV and other sources of hum.