Hello.  I'm going through the tutorial, and I've got to the part where I add sound, but no sound is forthcoming.  No errors or stack traces either.  Just silence.  I can use an audio player (mplayer) to play media/Beep.wav and I hear it, but when I run over the stars with the ship there's nothing.  alsamixer shows no muted output devices.

  OS: Debian testing ("squeeze")
  Kernel: 2.6.32-5-686
  Gosu: 0.7.25
  Ruby: MRI 1.8.7

I don't know a thing about audio on Linux.  Where should I start to diagnose this?
Thanks for any help you can offer.