You can increase your MPlayer’s soft volume to 300% by just typing this simple command at terminal:
mplayer -softvol -softvol-max 300 'file_name.avi'
and replace file_name.avi with the video file name you want to watch
Now use the “0” and “9” keys on your keyboard, to manually change the volume of MPLayer.
Please let me know if it did work for you…