Berkley,
Go to your CO Trunk Group, but it depends on what type of trunking you have on where you need to go.
System > Devices and Feature Codes > CO Trunk Groups > {trunk group ID}:
OR
System > Devices and Feature Codes > SIP Peers > SIP Trunk Groups > {trunk group ID} > Trunk Group Configuration:
Then just set the following:
Music-On-Hold: Audio Source <-This would be whatever audio source you are wanting to use.
Audio on Transfer to Ring: Music-On-Hold
By default your Audio on Transfer to Hold and Audio on Hold for Transfer Announcement are already set for Music-On-Hold.
Thanks,
TE