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 should work.
Not even going to ask why you can't use a keyboard or the system tray speaker icon.
man, win7 never stops surprising me, even after years of using it I still find flaws in it.. I gave up on this and made it a permanent item on the start menu. I did not want this item on my desktop, and I did find a way to do in in the control panel it required many clicks. Major PITA for a simple audio change.
The fact I cant even view the control panel is a detailed view says a lot about the incompetence of Microsoft and its programmers.