Hi Leif
You were spot on, sir!
I am indeed playing from a laptop, and when I opened the "Master Volume" control the "Mute all" box was checked... as I'd expected - I could hear music.
So I unchecked the box, but the music kept playing.
And now the usual button-press* to mute the audio works properly. All is back in sync.
Hurrah! Thanks for clearing it up.
Cheers
Steve
PS. Oh, and with regards to it being "unreliable design", it's a Dell XPS laptop... so you were pretty spot on there too.
(* - actually on my keyboard, but the laptop's AV controls button will do the same thing)