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 0
162 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.