Just bellow that line add:
amixer sset Master toggle amixer sset Master 5%+ amixer sset Master 5%- Then log in and out.If this don't work, then you can set a different shortcut to the that is guaranteed to work.
Here's what you need to know in order to set a shortcut. C = ctrl A = alt W = super Function keys = F1, F2, F3, etc. Example (Launched rhythmbox when the key combo "ctrl+alt+s" is pressed: rhythmbox