Want a quick way to set your system alert volume to a different level than your overall system volume? If you’re running OS X 10.4, here’s how to do it. (This trick works in 10.3, too, but with a minor caveat.)
First, you must have the Volume control icon (a small speaker) visible in your menu bar. If it’s not visible, open System Preferences, click on the Sound panel, and make sure that the Show volume in the menu bar checkbox is checked.
To adjust your system’s overall volume, you click once on the speaker icon in the menubar, and then move the slider on the drop-down control. If you just want to change the system’s alert volume, however, hold the Option key down when you click on the speaker icon. You can then release the Option key and move the slider as you normally would. Except now, you’ll only be changing the system’s alert volume, not the overall volume. If you have iTunes playing, for instance, you won’t notice any change in its volume level—at least if you’re running 10.4.
If you’re using 10.3, this trick works, but it will also adjust your system volume to the same level. So after Option-clicking to change the alert volume, click the widget once more and reset your system volume to your desired level (or just use the keys on your keyboard, if applicable). It’s a bit of a hassle, but probably still easier than opening the Sound System Preferences panel just to change the alert volume.