I am using the MOH (Music on Hold) 3.5 jack plugged into a bookshelf stereo system to play music and a voice over on hold. Currently the music can be hear but the voice over cannot be heard. They are part of the same track on a CD. Is there something I need to do to the system to pick up both layers of the track or do I need to make a new CD with the voice over "intertwined" with the music on the track?

you should use a 1/8th " mini MONO plug, stereo will not work. If your output from your HiFi is stereo, RadioShack makes a cord that goes from stereo (2 bands) to mono (1 band).