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.
It really works..