amixer worked for me but I didn't get the nice animation that I get when I press the volume up button on my keyboard.
I decided to use
xte to directly press that key from the command line:
xte 'key 0x1008ff13'
xte 'key 0x1008ff11'
xte 'key 0x1008ff12'
I figured out the keysym (that hex number) by using
sudo apt-get install xbindkeys xautomation
and then press the volume up button on your keyboard to get the keysym. The key sym may vary from system to system so finding it from xev will be the most reliable way.
The result looks like this for me:
KeyRelease event, serial 37, synthetic NO, window 0x2c00001,
root 0xef, subw 0x0, time 6660080, (566,573), root:(664,651),
state 0x0, keycode 123 (keysym 0x1008ff13, XF86AudioRaiseVolume), same_screen YES,
XLookupString gives 0 bytes:
XFilterEvent returns: False