Went to System Settings -> Sound and nada. Ubuntu Tweak does't appear in the Software Center anymore. How can one disable these dang bongos in 16.04?
The file that plays when the login screen is shown is:
Delete or rename it to prevent the sound from playing altogether, or replace/redirect it to make another sound play instead.
AFAIK this is not unique to 16.04, so this should also apply to all other versions of Ubuntu.
I'm Using Ubuntu 16.10. The answer is surprisingly simple. From the Login screen, mute the sound (the setting is positioned in the top right corner) and that's it. Muting the login screen sound does not affect your session sound. I suggest you also try this for other versions of Ubuntu before installing additional software or tweaks.
In both 14.04 and 16.04 /usr/share/sounds/ubuntu/stereo/system-ready.ogg is actually a link to dialog-question.ogg as seen below.
$ ll /usr/share/sounds/ubuntu/stereo/ total 340 drwxr-xr-x 2 root root 4096 Jul 22 2014 ./ drwxr-xr-x 3 root root 4096 Jul 22 2014 ../ -rw-r--r-- 1 root root 5016 Mar 2 2011 bell.ogg -rw-r--r-- 1 root root 8997 Mar 2 2011 button-pressed.ogg -rw-r--r-- 1 root root 4035 Mar 2 2011 button-toggle-off.ogg -rw-r--r-- 1 root root 4035 Mar 2 2011 button-toggle-on.ogg -rw-r--r-- 1 root root 104421 Mar 2 2011 desktop-login.ogg -rw-r--r-- 1 root root 26925 Mar 2 2011 desktop-logout.ogg -rw-r--r-- 1 root root 10660 Mar 2 2011 dialog-error.ogg -rw-r--r-- 1 root root 5377 Mar 2 2011 dialog-information.ogg -rw-r--r-- 1 root root 9851 Mar 2 2011 dialog-question.ogg -rw-r--r-- 1 root root 12217 Mar 2 2011 dialog-warning.ogg -rw-r--r-- 1 root root 22733 Mar 2 2011 message-new-instant.ogg -rw-r--r-- 1 root root 10429 Mar 2 2011 message.ogg -rw-r--r-- 1 root root 29299 Mar 2 2011 phone-incoming-call.ogg -rw-r--r-- 1 root root 7996 Mar 2 2011 phone-outgoing-busy.ogg -rw-r--r-- 1 root root 4792 Mar 2 2011 phone-outgoing-calling.ogg -rw-r--r-- 1 root root 17274 Mar 2 2011 service-login.ogg -rw-r--r-- 1 root root 14573 Mar 2 2011 service-logout.ogg lrwxrwxrwx 1 root root 19 Feb 26 2016 system-ready.ogg -> dialog-question.ogg -rw-r--r-- 1 root root 6994 Mar 2 2011 window-slide.ogg
This being the case modifying the link isn't really modifying a system file, it's modifying a pointer to a system file. It should be simple enough to point it elsewhere to the ogg file of your choice. for instance the slightly more pleasant service-login.ogg with
ln -sfT service-login.ogg system-ready.ogg or you could rename or delete the link nullifying it's usefulness but I would prefer to simply repoint it to something I might want to hear.
Testing on 14.04 and 16.04 with Unity DE.
I believe the appropriate way to do this would be with a sound theme. You could create a theme that plays a blank or silent .ogg file for this sound. See this answer regarding theme creation.
I found that this sound is played due to /usr/share/gdm/autostart/LoginWindow/libcanberra-ready-sound.desktop file, which is provided by gnome-session-canberra package. It says in description that it provides startup/shutdown sounds. Unfortunately, it's not possible to remove this package without breaking ubuntu-desktop package.
There are no knobs or settings to fix this issue.
The least invasive thing I found was to add X-GNOME-Autostart-enabled=false into /usr/share/gdm/autostart/LoginWindow/libcanberra-ready-sound.desktop file.
I'll file some bugs about this (package should be removable without dependencies break, and there should be some normal way to disable sounds through symlinks in /etc or separate settings).
I had disabled the login sound as suggested by Organic Marble, but I would still occasionally get a loud chime when I hit the splash screen. It was an absolute PITA to diagnose because it would only happen sometimes. I eventually worked out that it was happening when I had connected or disconnected my power cable while the laptop was suspended AND if the laptop sound wasn't muted. When the laptop reactivated, it noticed the (dis)connect at the same time the splash screen came on.
I couldn't find a setting anywhere that would disable that chime, so I ended up going with a variant of Ben Leggiero's answer and renaming files:
/usr/share/sounds/KDE-Sys-Log-In-Long.ogg /usr/share/sounds/KDE-Sys-Log-In.ogg /usr/share/sounds/KDE-Sys-Log-In-Short.ogg /usr/share/sounds/Oxygen-Sys-Log-In-Long.ogg /usr/share/sounds/Oxygen-Sys-Log-In.ogg /usr/share/sounds/Oxygen-Sys-Log-In-Short.ogg
Create a backup of the sound, delete it and replace it with an empty file
cd /usr/share/sounds/ubuntu/stereo/ sudo cp system-ready.ogg system-ready.ogg.back && sudo rm system-ready.ogg
To turn it back on
cd /usr/share/sounds/ubuntu/stereo/ sudo cp system-ready.ogg.back system-ready.ogg && sudo rm system-ready.ogg.back