If you are using any of the major distributions and if you are using GNOME or KDE, there are graphical interfaces that will allow you to specify hotkeys for this. If not, or if you want a generic solution, most distributions come with the
xlock command. You can then write a short script like this
amixer sset Master mute
Save this script somewhere in your
PATH (if you don't know what that is, look at this intro page on it) and make it executable with the command
chmod a+x <script name>.
Then, you'll have to set a hot key that will execute this script in your window manager, but that will depend on which one you're using. To find out more you can just Google your distribution or window manager's name along with the words "key bindings" or "hot keys" and you're likely to find instructions.
Unlocking from xlock is just a question of moving your mouse or pressing any key, and then entering your password.
If you want a different screensaver, run
man xlock and look at the various options.