Comment 10 for bug 201391

Please ignore /etc/acpi/ stuff, this is pretty obsolete, and faking keys is known broken. Apparently the headset's keys actually work, so it's a question of changing the real mixer level.

As far as I know, the kernel side of this is done once it correctly determines the keypresses and forwards them to userspace as input events. Luke, Daniel, do you know which part of the stack is/should be responsible for changing mixer levels like that? pulseaudio? GNOME mixer applet?