I've got a dual-core Opteron, QuickTime 7.5, Firefox 2.0.0.14.  Good luck figuring out when the CPU pegging, memory leaking Firefox 2 loaded QuickTime.

Given the ubiquitousness of Flash video players this isn't a tip that you'll be able to use too often, but when you're watching a video with miserably low audio it's a lifesaver  If you're watching a QuickTime movie press and hold the Shift key and then click on the volume control.

Normal Volume Control

Volume Control with shift key depressed