From the sounds.tbl:
$Name: 0 trk-loop.wav, 0, 0.40, 0 ; Missle tracking to acquire a lock (looped)
  You are getting a beeping sound because you referenced a beeping sound.
Try this:
+Num Intercom Sounds: 1 ;; MAX is 10, if you need more, you need to get a programmer
+Intercom delay: 8000 15000 ;; min and max random delay for intercom sound 0
+Intercom sound: 162 ;; SND_MAIN_HALL_INT1
+Intercom pan: 0.0 ;; pan for intercom sound 0162 is an empty slot in the default sounds table.  If you've put anything in that space in your customised sounds.tbl, you'll have to change it to refer to a different one.