Have a working 1a2 system and have recently noticed my cards say "Brand Rex LC-400E Music on hold"

I really want to get the moh feature working. Where the heck do i connect my music source? I installed this system so I have a little knowledge about it. I am by no means a pro like some of you though. The only info I found on my ksu was "55IC*KSU" I currently use 3 lines, The ksu has 4 cards in it only 3 of them are the Brand Rex Music ones.

Any info or help would be great.

Thanks

Mope