How do you connect Music to Avaya Partner ACS & Lucent phone system while on hold?

I don't really have a ton of knowledge about small business phone systems. I am attempting to play royalty free music over our phone system while we put people on hold. We have an Avaya (old school Lucent) Partner ACS system with 1 main unit and 2 extension units. It is R 4.1 version and we are currently running 4 lines off it and 10 extensions. There is a "Music on Hold" RCA jack in the front of the main unit. Right now we have an RCA cable attached to that from a computer where we are playing music to get it working. We hear nothing and have tried numerous things like making sure the feature is programmed on a phone, etc. Any help or ideas? Do we need a music coupler to make this work or a smaller watt source? Thanks for your help!