A few more updates here.  So looking at the Open Firmware screenshots I posted earlier, the only thing in the .properties for the sound device was "name pseudo-sound".  That doesn't match what the Linux driver expects.  So I reset Open Firmware (reset-nvram, set-defaults, and then reset-all) and found that the full device showed up, as AOAKeylargo and with the expected layout-id.  So something is happening where the OS 9 audio driver removes the sound device from the device tree.

As a test, I removed the Apple Audio Extension, and while the onboard output doesn't show up in the Sound preference panel any longer, sound output still works, and the Open Firmware sound device still ends up renamed after a reset.  So perhaps the sound driver built into the System file is doing this?

Also, any ideas on where to set a breakpoint so I'll drop into MacsBug when an application plays a sound?  I tried "tvb SndPlay" and it sets the breakpoint, but I never see it hit.