Thanks to kilowatt -- my speakers were indeed plugged into the mic jack, so the OS didn't see speakers, and didn't offer to output signal to them. I plugged them into the line-out jack, and now all is well.
Thanks in advance for suggestions --
I've a PowerMac G5 dual 2.5 GHz running Tiger 10.4.2. My preference panel for sound does not show an option to select line-out for output, while System Profiler shows the line-level output as available in the Audio (Built In) section.