1. Ensure that the wires in place are securely connected at both ends, and free of breaks, fraying, or missing insulation.
2. Make sure the file is formatted correctly. Recommended MP3 parameters are Mono, 128k Bit Rate (kbs), 44 KHz (44100 Hz) Sample Rate
3. Test via an outside line; some phone systems will only play MOH over outside lines.
4. Reboot unit and see if audio plays after.