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!