I'm playing at fullscreen and often working in parallel. Hence I often switch from the game. I rarely could hear music track until the middle.
It is even a bit disappointing. A lot of awesome work was done but I can't hear it through my usual gameplay.

Why even stop the music when you change windows?

Because when you change windows, you're multi-tasking, and your task may require silence.

It should be a preference as to whether it auto disables the audio on focus loss, imo.

Definitely this should be available in the preferences menu rather than automatically on, but I think it was a classy touch.
That said, I completely concur with the OP. I also tend to ADOM while multitasking so having a track restart repeatedly is irritating. Especially at the start of the game when that beautiful intro track is JUST getting going, it'd be really nice for it not to restart because I clicked the wrong part of the screen but simply to pause and continue when switching back?

Please make this an option in the game. It doesn't even have to restart at the point where it left off. An option to have the music keep playing on alt-tab would be fine as well in my opinion.

Still nothing after almost a year? An option to keep the music playing while alt-tabbing really doesn't seem very difficult to implement :/

Pause/resume implemented (in NotEye).