There is no wait or delay statement in standard COBOL. There may be, for GnuCOBOL, a CALL "SYSTEM" to effect a delay.
I took some code that I use for elapsed time measurement and modified the code to create a procedure for a delay.
Wherever you need a delay, insert the statement PERFORM timed-delay. Of course, the delay may be changed. This code is set to work even if the delay crosses midnight.
Code:
   working-storage section.
   01 t pic 9(8).
   01 t-start.
     03 t-start-hour pic 99.
     03 t-start-minute pic 99.
     03 t-start-second pic 99v99.
   01 t-end.
     03 t-end-hour pic 99.
     03 t-end-minute pic 99.
     03 t-end-second pic 99v99.
   77 t-elapsed pic 9(7)v99.
   procedure division.
   begin.
       accept t from time
       display t
       perform timed-delay
       accept t from time
       display t
       stop run
       .
   timed-delay.
       accept t-start from time
       move 0 to t-elapsed
       perform until t-elapsed > 0.5   *> one-half second
           accept t-end from time
           perform get-elapsed
       end-perform
       .
   get-elapsed.
       if t-start > t-end
           move 86400 to t-elapsed
       else
           move 0 to t-elapsed
       end-if
       compute t-elapsed = t-elapsed
         + (t-end-hour - t-start-hour) * 3600
         + (t-end-minute - t-start-minute) * 60
         + (t-end-second - t-start-second)
       end-compute
       .
Output: (shows a delay of 0.55 seconds)
21424364
21424419
The initial PERFORM WITH TEST AFTER ... is nothing like the code I provided in: Cobol-Restart from the program , so I turned it into comments. It should be removed.
If you want to use SLEEP-SEC instead of a fixed value, replace the 0.5 with SLEEP-SEC; but provide a VALUE clause for SLEEP-SEC or MOVE a value to it before the displaying the menu.
For example, in your code (with most code removed):
   DATA DIVISION.
   WORKING-STORAGE SECTION.
   01  SLEEP-SEC PIC S9(2)V9(2).
   01  A PIC S9(7)V9(7).
   01  B PIC S9(7)V9(7).
   01  C PIC S9(7)V9(7).
   01  D PIC S9(11)V9(7).
   01  INPUT1 PIC 9(14).
   01  Q PIC X VALUE "Y".
   01 t-start.
     03 t-start-hour pic 99.
     03 t-start-minute pic 99.
     03 t-start-second pic 99v99.
   01 t-end.
     03 t-end-hour pic 99.
     03 t-end-minute pic 99.
     03 t-end-second pic 99v99.
   77 t-elapsed pic 9(7)v99.
       PROCEDURE DIVISION.
   MAIN.
  *    PERFORM WITH TEST AFTER
  *        UNTIL Q ="YES" OR "Y" OR "y" OR "yes" OR "Yes"
  *    END-PERFORM.
       DISPLAY "CALCULATOR".
       DISPLAY "WHAT DO YOU WANT DO DO?".
       DISPLAY "1 ADDITION".
       DISPLAY "15 EXIT"
       DISPLAY "CHOOSE AN OPTION"
       ACCEPT INPUT1
       EVALUATE  INPUT1
       WHEN = 15
            DISPLAY "OK, GOOD JOB :)"
            STOP RUN
       WHEN = 1
       DISPLAY "FIRST NUMBER"
       ACCEPT A
       DISPLAY "SECOND NUMBER"
       ACCEPT B
       COMPUTE C= A + B
               DISPLAY "Computing"
       PERFORM timed-delay
               DISPLAY "(" A ")" "+" "(" B ")" "RESULTS..."
               DISPLAY C
       END-EVALUATE
               IF INPUT1 NOT = 15
           DISPLAY "DO YOU WANT TO DO OTHER CALCULATIONS?"
           ACCEPT Q
           IF Q = "YES" OR "Y" OR "y" OR "yes" OR "Yes" GO TO MAIN
               ELSE DISPLAY "OK, GOOD JOB :)"
              END-IF
              STOP RUN.
   timed-delay.
       accept t-start from time
       move 0 to t-elapsed
       perform until t-elapsed > 0.5   *> one-half second
           accept t-end from time
           perform get-elapsed
       end-perform
       .
   get-elapsed.
       if t-start > t-end
           move 86400 to t-elapsed
       else
           move 0 to t-elapsed
       end-if
       compute t-elapsed = t-elapsed
         + (t-end-hour - t-start-hour) * 3600
         + (t-end-minute - t-start-minute) * 60
         + (t-end-second - t-start-second)
       end-compute
       .