As the driver level could be the problem, I updated to 12.16 (was using AN 12.14)
Following a reboot there was still no sound
I also downloaded USBAUDIO-12.16.exe which unpackaged itself into \MMOS2\INSTALL\usbaudio but did not install any files anywhere. I replaced the usbaudio.sys/sym files in \mmos2 with the files from \MMOS2\INSTALL\usbaudio and rebooted.
'USBAUDIO-12.16.exe' is the old IBM driver that lacks Lars' additional features. Do not intall it. Also, *.sym exist solely to make debug reports more intelligible. They are completely passive (and generally unused), so copying in the wrong ones isn't helping anything.
The Desktop Startup sound almost played - started but cut off about 66% of the way through.
So, you rebooted and got no sound, then did something that had no effect and rebooted. Voila, sound! Are you sure you didn't do anything else?
Do I need to "clean up" the mmos2 setup?
Messing-around when you don't know the correct configuration is seldom helpful.
I have the files from the Enhanced USB audio packages installed although I am not using the drivers from the package.
What does this mean? If the Enhanced USB audio packages are installed, how can you not be using its drivers?
At this point, I can only think of two possibilities:
* unlikely: maybe this is a USB Audio v2 device. With no audio streams open, set "UAC2 Adapter Wave" as the default in MM Setup, close it, then play a sound. If this fails, go back to "USB Adapter Wave".
* likely: perhaps there are still problems with xHCI isochronous transfers. File a bug report.