Under Mac OS 9, the Sound control panel has several new aspects to it, one being the ability to check your speaker setup when you have either headphones or speakers plugged into the speaker port.
The picture of the headphones is a representation for any device connected to the speaker port of the computer. The Sound control panel will always show the headphones picture no matter what device you have plugged into the speaker port.
The same does not hold true for speakers hooked up to the USB port. For example, the speaker setup will not only see USB speakers but it also has the ability to see USB sub-woofers, if they are attached.