Okay. So I figured it out. I was having issues with the Silverlight plug-in not playing sound in Firefox. According to the help info, one suggestion was to go to Control Panel. And use the volume mixer there to check the sound volume. (I tend to use the system tray applet to control my volume, bypassing the full mixer.)
Next to the system volume, I saw that the volume slider for Firefox was muted... I hit the mute button to enable the volume. Lo and behold, the sound now plays fine.