Type in command line
emerge xmms-status-plugin
Next, if your XMMS is running, we need to restart it to make plugin available.
Configuring plugin
1. From XMMS menu choose Options->Preferences or press Ctrl+P to access preferences
2. Choose "General Plugins" tab and you should see Status Docklet
3. Select the plugin and press "Configure"
4. Defaults are fine but change them how ever you prefer
5. In the "Button actions" I recommend you keep "Toggle All Windows" on for some button so you can pop the XMMS from the systray
6. Press "Ok"
Now you have your plugin configured.
Configuring KDE
Now we need to tell KDE to hide XMMS from taskbar.
1. Open KDE Control Center
2. Choose Desktop
3. Choose Window-Specific Settings
4. Press "New"
5. Press "Detect" button under Detect Window Properties
6. Your cursor should change into crosshair. Press XMMS.
7. You've detected XMMS properties. Make sure you select "Use Window class (whole application)"
8. Press "Ok" and change to Preferences tab
9. Select "Skip taskbar", select Apply Initially from dropdown list and select the cross on the right side of dropdown list.
10. Press "Ok"
You're done! Restart XMMS to apply changes.
P.S. It's a pretty nice explanation on the web which I gave you! From time to time it's not bad to read it

) It says:
Using
After installing the plugin, you need to add a "Status Dock" to your GNOME panel. From the Foot Menu choose Panel->Add to Panel->Status Dock.
Now open up the XMMS preferences window (right click on the XMMS window and choose Options->Preferences from the resulting menu). On the tag "Effect/General Plugins" there will be an entry for the Status Docklet Plugin. Select it and click "enable plugin" the word enabled will appear after the Status Docklet entry and the status docklet will appear on your panel.
Clicking on the status docklet icon shows/hides the main window by default (this can be changed from the status docklet preferences), and right clicking gives a menu of commands.