Well for system volume u can increase the Master and PCM volume by typing cmd on terminal-
it opens alsamixer & u can set max volume with the help of keyboard controls.
for playing audio & video files, Get smplayer by typing following on terminal-
$ sudo apt-get install smplayer
If you already install SMPlayer, you can increase the volume. Just click preferences button and then you can configure it. On Preferences window, just go to General | Audio tab. Check “Use software volume control” and set your maximum amplification.
mplayer also has this feature of software volume amplification,but the gain is not so much...so plz use smplayer...
It really works..