If you want to make use of the volume up/down buttons on your X201, you can simply install xbindkeys and add the following to your ~/.xbindkeysrc:
#VolDown
"amixer -q set Master 5%-"
m:0x0 + c:122
NoSymbol
#VolUp
"amixer -q set Master 5%+"
m:0x0 + c:123
NoSymbol
Beware: The keycodes are different on the X200 and the X201! For the X200, use the following configuration:
#VolDown
"amixer -q set Master 5%-"
m:0x0 + c:174
NoSymbol
#VolUp
"amixer -q set Master 5%+"
m:0x0 + c:176
NoSymbol
For testing, you can launch
xbindkeys --verbose -n
Afterwards, you can include xbindkeys into your .xinitrc without any parameters, as it automatically forks into the background:
xbindkeys
The volume mute button also generates an event on the X201 (none on the X200), but it seems like muting is done in hardware, so no need for a mapping.
Have fun!