Mapping complex key combination to Volume Down/Up in Karabiner
You have three key codes in each definition and you use one modifier key (COMMAND_L) as a key code - which won't work as expected. Instead the following will be executed hitting Shift_LAlt_LCtrl_L+Cmd_L (you have to use the Command_L key as a normal key):


A left bracket will be written to a text field  


and  


Volume down (one level)


The second part of your command (Volume up) will be skipped completely because the key code is essentially the same as the first one and thus already in use:

KeyCode::COMMAND_L, ModifierFlag::OPTION_L | ModifierFlag::SHIFT_L | ModifierFlag::CONTROL_L,


If you hit and hold the Command_L key first and then hit and hold all other modifier keys and one of the brackets nothing will happen at all because Command_L needs a key down and up to be executed in your current configuration!



So use the following snippet instead:

&lt;item&gt;
    &lt;name&gt;Hyper Bracket Volume Control&lt;/name&gt;
    &lt;appendix&gt;Hyper+[ and Hyper+] Volumn Down/Up&lt;/appendix&gt;
    &lt;identifier&gt;volume.hyper&lt;/identifier&gt;
    &lt;autogen&gt;
        __KeyToKey__
        KeyCode::BRACKET_LEFT, ModifierFlag::COMMAND_L | ModifierFlag::OPTION_L | ModifierFlag::SHIFT_L | ModifierFlag::CONTROL_L,
        KeyCode::VK_CONSUMERKEY_VOLUME_DOWN
    &lt;/autogen&gt;
    &lt;autogen&gt;
        __KeyToKey__
        KeyCode::BRACKET_RIGHT, ModifierFlag::COMMAND_L | ModifierFlag::OPTION_L | ModifierFlag::SHIFT_L | ModifierFlag::CONTROL_L,
        KeyCode::VK_CONSUMERKEY_VOLUME_UP
    &lt;/autogen&gt;
&lt;/item&gt;