You can control your playback sound levels from the control panel sound applet.
So without having access to the keyboard or the system tray, you should be able to launch the control panel by clicking the start orb and selecting it. If not the add then control panel icon to your desktop, by right clicking the desktop and choosing personalize and then the change desktop icons link and checking the box next to Control Panel.
Change the View by: drop down in the top right from category to small icons.
You can either use the native sound applet or if your sound card placed another applet you can use that.
You can also right click the applet and choose create shortcut and it will place a shortcut on your desktop.
Kinda cumbersome but it...