To be able to control the mixer/pcm volume with the multimedia keys on your keyboard/notebook while using the fluxbox window manager, just add the following lines to your \~/.fluxbox/keys :

None XF86AudioLowerVolume :ExecCommand amixer -q set Master 5%- unmute
None XF86AudioMute :ExecCommand amixer -q set Master toggle
None XF86AudioRaiseVolume :ExecCommand amixer -q set Master 5%+ unmute

and restart or 'reconfigure' fluxbox (you can do it from the main fluxbox menu).