Some keys are not detected in my keyboard (multimedia keys?) by either xev or xbindkeys or keytouch: how to detect them?

You may need acpi support

sudo apt-get install acpi acpi-support acpica-tools acpid acpidump acpitail acpitool libacpi0 laptop-detect pommed

Beyond installing the above packages, I finally fixed this issue on my laptop by uninstalling the toshet package after discovering my specific model was not compatible or supported by the toshet application. I doubt you have toshet installed but you can check to see by typing the following command:

dpkg -l | grep toshet

There should be no output if toshet is not installed. If it is listed in the subsequent output of that command, you can uninstall it by typing the following:

sudo apt-get purge toshet