Eu meio que resolvi um problema semelhante usando este link :
amixer set Master 1+
amixer set Master 1-
amixer -D pulse set Master toggle
Resta vincular os comandos com as chaves. No meu caso, com o OpenBox, usei o seguinte em .config/openbox/rc.xml
:
<keyboard>
...
<keybind key="XF86AudioRaiseVolume">
<action name="Execute">
<command>amixer set Master 1+</command>
</action>
</keybind>
<keybind key="XF86AudioLowerVolume">
<action name="Execute">
<command>amixer set Master 1-</command>
</action>
</keybind>
<keybind key="XF86AudioMute">
<action name="Execute">
<command>amixer -D pulse set Master toggle</command>
</action>
</keybind>
</keyboard>