You might encounter a problem that when you start the kmix sound mixer you would not see any thing happening. It happens because it runs as an hidden application, so to access it you would need to add the system tray widget.

To add the system tray widget to your panel perform the following steps:-

1. Right click on the panel that you want to add your widget to.

2. Select the system tray widget as shown in the figure below.

3. Now you will be able to access the all the hidden applications and kmix being one of them, you would get the sound control applet where you can control the volume functionality.

Note: In case the above solution does not work you may try upgrading the kmix package by executing the following command.

sudo apt-get upgrade kmix