A MacFixIt staff member reports that when moving objects around with multiple displays connected (using a second video card), the sound output of his Mac exhibits "scratchiness" and distortion. Specifically, if iTunes or QuickTime player is playing audio, when moving screen objects on a second (non-menubar) display in a way that requires significant redrawing of the screen -- especially when resizing windows in certain applications -- the audio becomes "scratchy" or distorted during the drag, and remains that way even when the dragging stops. Pausing iTunes or QuickTime player and then starting it again fixes the problem (until the next time the problem occurs).
The machine in question is a Power Mac G5 Dual 2GHz with a stock AGP Radeon 9600 and a PCI Radeon 7000, each with a single monitor connected. However, this is evidently not the same issue with "noisy" G5s reported previously on MacFixIt, as it only occurs when objects are resized/moved on the secondary display. The computer in question has the latest drivers for both video cards installed.
One obvious difference between the two displays is that the main screen, run from the AGP Radeon 9600, is Quarz Extreme-enabled, whereas the second screen isn't Quartz-enabled. However, we don't know if this fact is related to the problem described here.