Yeah, titi mentioned to me in IRC yesterday that it was intentional.
But there's already a "cavalry charge" sound when someone joins the lobby or a server becomes available. I'm not sure what the point is in having an option to mute the background music if it auto-disables itself randomly.
I sometimes am not at my computer, but nearby it and don't want the music on all the time.
I usually have my MG music level volume to 0 to work around it.