For Ubuntu 12.04+

LightDM Way

Also known as the FluteFlute Slap, the LightDM Way involves having to logout and on the LightDM Login Screen you can change the volume or even mute the sound.

In the Login Greeter you can see the Volume on the top right. Adjusting the volume there does not affect the volume when the session starts, so you can MUTE the sound in the Greeter without muting the sound when your session starts:

Ubuntu Tweak Way

The Ubuntu Tweak way involves having to download the Ubuntu Tweak app and then disabling the sound from there. Do the following steps and you can do it this way:

sudo add-apt-repository ppa:tualatrix/ppa
sudo apt-get update
sudo apt-get install ubuntu-tweak

Open Ubuntu Tweak and on the Tweak Tab select "Login Settings" option, you should see the Play Login Sound option. Yo need to click on the Lock button above to be able to change this.

For Ubuntu 11.10 and Below

Go to System -> Preferences -> Startup Applications

Disable the one called GNOME Login Sound

(uncheck the box and click close)

you can also go to System -> Preferences -> Sound and disable the whole sound system (Like windows sounds, alerts, etc..)

In older version you could go to the Sound Preference part and select each sound and what you wanted to do. It was removed around 9.04 or 9.10. It would be good if it came back.

The start-up sound can be disabled editing the Unity Greeter configuration. Running the following from a terminal will do this:

echo -e '[com.canonical.unity-greeter]\nplay-ready-sound = false' \
    | sudo tee -a /usr/share/glib-2.0/schemas/50_unity-greeter.gschema.override
sudo glib-compile-schemas /usr/share/glib-2.0/schemas/

If you are running Ubuntu 12.04 LTS make sure you have updates installed as this feature was added in an update.

12.04 - 16.10 (LightDM)

Here is what worked for me after I installed 12.04 last week.

The steps are these, adapted from http://www.liberiangeek.net/2012/05/disable-the-ready-drum-sound-in-ubuntu-12-04-precise-pangolin/:

sudo nano /usr/share/glib-2.0/schemas/50_unity-greeter.gschema.override

Add these lines:

[com.canonical.unity-greeter]
play-ready-sound = false

Save, then run:

sudo glib-compile-schemas /usr/share/glib-2.0/schemas/

Reboot and enjoy.

Edit: gksu and gksudo were removed in 18.04 but the linked article describes how to edit system files without them. I have updated the first terminal command to demonstrate how one might edit the file from the command line. Replace nano with the editor of your choice.

11.10 (LightDM and GDM)

Quick solution:

Open a terminal and run the command

sudo mv /usr/share/sounds/ubuntu/stereo/desktop-login{,-disabled}.ogg  

The lightdm drum sound for system ready can be disabled in a similar fashion

sudo mv /usr/share/sounds/ubuntu/stereo/system-ready{,-disabled}.ogg  

For 12.04

By default, it is turned off. To enable it, open a terminal and run

gksudo gedit /usr/share/gnome/autostart/libcanberra-login-sound.desktop

Then change the value for X-GNOME-Autostart-enabled to "true", i.e. X-GNOME-Autostart-enabled=true

For 11.10

Type on a terminal gksudo gedit /usr/share/gnome/autostart/libcanberra-login-sound.desktop and change the value NoDisplay from true to false:

Now go to your startup applications and you should see and be able to disable the necessary option:

10.10 (GDM)

There appears to be a bug in the Login Screen about this. I can't find an official report, however, this forum post confirms a fix.

Go to System/Administration/Login Screen. Unlock it, then tick "Play login sound".

Close the window, then, go back into the app again, unlock it, then untick the same option.

If you are using GDM with 12.04

To disable the login sounds, the event-sounds key under org.gnome.desktop.sound for the gdm user must be set to false.

For this, we must first give GDM ownership of its own home directory. Open a terminal, and type: sudo chown -R gdm:gdm /var/lib/gdm

Then run:

sudo xhost +SI:localuser:gdm
sudo -u gdm gsettings set org.gnome.desktop.sound event-sounds false

and logout (or reboot).

If you are using LightDM with 12.04

Open a terminal with Ctrl+Alt+T, and type/paste the following, one line at a time:

sudo xhost +SI:localuser:lightdm
sudo su lightdm -s /bin/bash
gsettings set com.canonical.unity-greeter play-ready-sound false
exit

What this does is disable the play-ready-sound setting for the login screen/desktop manager user (lightdm).

11.10 (LightDM and GDM)

For removing the startup sound :

mv /usr/share/sounds/ubuntu/stereo/system-ready.ogg /usr/share/sounds/ubuntu/stereo/system-ready.ogg.old 

For enabling the startup sound

mv /usr/share/sounds/ubuntu/stereo/system-ready.ogg.old /usr/share/sounds/ubuntu/stereo/system-ready.ogg

For removing the Login Sound (After you login) :

mv /usr/share/sounds/ubuntu/stereo/desktop-login.ogg /usr/share/sounds/ubuntu/stereo/desktop-login.ogg.old

For enabling it again :

mv /usr/share/sounds/ubuntu/stereo/desktop-login.ogg.old /usr/share/sounds/ubuntu/stereo/desktop-login.ogg

11.10 (GDM)

gsettings set org.gnome.desktop.sound event-sounds false

11.10 (LightDM and GDM)

This should work for you.

gksudo gedit /usr/share/gnome/autostart/libcanberra-login-sound.desktop

Then change the NoDisplay= to false, ect.

or to remove it:

sudo rm /usr/share/sounds/ubuntu/stereo/system-ready.ogg