Even if any music player like mplayer is used to activate the subwoofer channel, It won't get activated until 6 channel mode is activated in the volume control dialog.
To do this first double click the volume control icon on the top panel.
On the volume control dialog check if an "options" tab is available.
If not then click the "preferences" button and tick the "options" check box to activate it.
Now in the "options" tab change "Channel Mode" to 6 channel.
My sound driver & Mixer is ALSA for nVidia.