Switching between speakers and headphones.

On my new Acer laptop with Vista Business I can start playing a music file
and of course it comes out through the speakers. I then plug in the
headphones, expecting
a. the speakers to go silent
b. the music to come through the headphones.

But wmp takes no notice of the headphones and continues to play through the
speakers.

BUT if I plug in the headphones, THEN start WMP, the sound comes through the
headphones. BUT if I then disconnect the headphones, expecting the sound to
revert to the speakers, I get a message saying "An audio device was
disconnected...." and I have to start the track again (at which point it
reverts to the speakers).

Is all this "AS DESIGNED"???