Many times I'm listening to a QuickTime movie with headphones and the volume isn't loud enough, even with the system volume maxed out and QT set to maximum. And with the system sound maxed, every other system sound blasts in my ears.
I found this little tidbit by accident ... choose Movie -> Get Movie Properties. In the new window that opens, select Soundtrack on the left pop-up menu and Volume on the right. From there, you can increase the gain by dragging the green bar in the Volume section, as seen in the image at left (click on it for the larger version).
I'd love to be able to do this in DVD Player, too, because I have trouble hearing some DVDs.
[robg adds: This appears to be how you can effectively match yesterday's hint about doubling the playback volume in browser-embedded QuickTime controllers. I've been using QT Player, as well as the Movie and Soundtrack settings, for quite a while and I'd never thought to try clicking in the empty area of the Volume box...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20041101115551841