Hey there, here is a quick snipped of how I accomplished a "Grand Theft Auto" style Radio Manager / Playback sources that maintain sync, constant song "playback" across all active channels (even when not being listened to) and more.
It's a pretty rudimentary system right now, and there are probably some edge cases and bugs that could be fixed. It should definitely get your tunes playing though!
Put RadioManager.cs on a GameObject in your heirarchy. I like to make a GameObject called Managers, and then make a separate GO for each Manager I have. Then put RadioPlayer.cs on an object that you want to have playback. It requires an AudioSource.