Somehow the problem resolved itself after I restarted the game completely the next day.
To answer you question, the song is a custom one.
Both clients had the song file in the correct location but for some reason I (as the editor of the base at the time) did not have the song play properly.
I was able to switch songs for other players in the base including the custom song, but I would never hear it, only the canned songs.
Everything is working fine now.
So looks like the restart did the trick.
<edit> Typo