Bell (X11)
The X protocol's only sound feature: a request asking the server to beep, at a volume given in percent.
The core protocol has no audio support at all, just the Bell request with a volume between −100 and 100 relative to the base setting. Base volume, pitch and duration are set with ChangeKeyboardControl — the beep was treated as a property of the keyboard, which on many early workstations is physically where it lived. From the shell, xset b does the same, and xset b off has silenced more open-plan offices than any other command.
When a terminal receives the BEL character, Ctrl-G, it calls XBell, or flashes instead if xterm was started with visualBell. XKB turned the beep into something more in 1996: named bells and the XkbBellNotify event, so a client can intercept the call and play a themed sound or flash a title bar rather than have the server beep the speaker. That is how desktop environments managed "system sounds" on top of a protocol that could only say beep.