I have this customer with a Nel Tech iLink Pro music on hold player with ethernet connectivity. Can the customer use it as a stand a lone player without a subscription? At the moment it doesn't play anything. We don't need much, just a basic message with music.

Remember that when you purchase the subscription to play MOH the music is licenced, thus the royalties to ASCAP, BMI are paid. The use of copyrighted material is not for use in the public domain. Even using your local radio station for rebroadcasting requires licencing. You need to find Royalty-Free music to be legal.

I have some royalty free music files that I would like to load on to this unit. Is it possible?
How is that for an answer Mr. Attorney General?

State or Federal...which has better bennies?

In looking at the diagram it looks like you could use a CD player on continuous play with the headphone output to Channel A input then Channel A output to the MOH input on the system. You would just need to get the appropriate adapters. Also a word of WARNING: Use Mono cables and not Stereo.