How can I automatically mute the volume at every boot?

I think this needs two parts:

1. To mute the sound in GDM, put:

   amixer sset Master 0
   amixer sset Master off
   

   in the file /etc/rc.local (make sure exit 0 is the last line in that file). This will set the volume level to 0 on the Master channel and mute it.

2. To mute the sound in your session, add the above to the startup applications:

   System > Preferences > Startup Applications, then click on Add and create the new startup entry:

   • Name: Mute Sound
   • Command: /bin/bash -c "amixer sset Master 0 ; amixer sset Master off"
   • Comment: Set Master channel volume to 0 and mute it

   Click on Add and Close, and you should be done.

You can skip part one, if you turn off the login sound in System > Administration > Login Screen (untick the Play login sound checkbox).

Mute the volume and then run the following command:

sudo alsactl store  

This will store your current sound settings, so from now on the sound will be muted on startup. I tested this out and it mutes both the GDM sound and the login sound.