Q:

Diary appoinment variables is void:

Reference to 31.10.6 Diary Appointments

If you have a diary entry for an appointment, and that diary entry
  begins with a recognizable time of day, Emacs can warn you in advance
  that an appointment is pending. Emacs alerts you to the appointment by
  displaying a message in your chosen format, as specified by the
  variable appt-display-format. If the value of appt-audible is non-nil,
  the warning includes an audible reminder. In addition, if
  appt-display-mode-line is non-nil, Emacs displays the number of
  minutes to the appointment on the mode line.

it mentioned the variables of appt-display-format, appt-audible and appt-display-mode-line,
Unfortunately, I cannot find the variables with prefix of appt 
eval (appt-audible) but  report :
 Symbol’s function definition is void: appt-audible
Why I cannot not find the variables of appointments?

A:

The library appt.el contains the variables/functions sought.  Consider using something like (require 'appt) before attempting to locate (with functions such as describe-... or find-...) and/or before attempting to use the variables/functions.
Q:  How did I find the answer to this question?
A:  I grepped the lisp folder (of the Emacs source code) for the prefix appt-, and/or, grepped for a random variable described in the Emacs manual (to which a link was provided in the question by the O.P. above); e.g., appt-display-format.

