Hold tones are played to callers who are put on hold or are in transfer. The On Hold setting in Telephony/Global settings/Feature settings, needs to be changed if you want callers to hear the external music source which you have connected to the 3.5mm socket at the front of the BCM.
Please note that any Music on hold facility added to the System will play for any calls on hold including calls waiting to be answered by the Contact Centre. You need a music source, usually a CD player or a Music on Hold device.
The music source can also be played as background music (Feature 86) on sets which are not busy, providing the Background Music box is ticked. Several other settings can also be changed in the feature setting menu including the Held Line Reminder Time; Transfer Callback Timeout etc. A mouse over will give a brief description of each setting.