Do you want to mute and unmute the system volume simply by pressing a key combination ?
You can easily do that with NirCmd utility. Just download NirCmd ZIP file from here, extract NirCmd.exe to any folder you like, and then run NirCmd with the following command-line parameters:
nircmd.exe cmdshortcutkey "~%folder.desktop%" "Mute Volume" "CTRL+SHIFT+M" mutesysvolume 1
nircmd.exe cmdshortcutkey "~%folder.desktop%" "Unmute Volume" "CTRL+SHIFT+U" mutesysvolume 0

After executing the above 2 lines, you'll find 2 new shortcuts on your desktop: "Mute Volume" and "Unmute Volume". Pressing "CTRL+SHIFT+M" will activate the "Mute Volume" shortcut and will mute your system volume. Pressing "CTRL+SHIFT+U" will activate the "Unmute Volume" shortcut and will unmute your system volume.
The CTRL+SHIFT+M and CTRL+SHIFT+U key combinations are just an example, and you can choose any hot-key combination you like, as long as the key combination you choose is not registered by Windows operating system or by other software.

If you want to mute/unmute your system volume by using only one key combination (the first key press will mute the volume and the second key press will unmute it), just try this example:
nircmd.exe cmdshortcutkey "~%folder.desktop%" "Switch Volume" "CTRL+SHIFT+S" mutesysvolume 2

...And finally, whenever you wish to remove these hot-key combinations from your system, simply delete the shortcuts created by NirCmd from your desktop.

Update: There is also a newer and easier method to create a shortcut to mute/unmute, using the SoundVolumeView tool:
Create shortcut to mute, unmute, or change sound volume of audio devices and applications