For the second time, Windows 10 Update has automatically installed a "critical update" that causes the touch screen and audio to fail!
First time I managed to do a System Restore to get back to functionality, and when it installed again I re-ran the original Intel Atom Z3000 series driver set (HP file sp72926 signed Oct. 2015), which restored functionality while allowing the Windows Update.
The second time (25 June 2016), I tried reinstalling the drivers but this time it aborted and forced a rollback, leaving me without touch or sound.
Finally I hit upon a solution! Device Manager revealed that the I2C drivers were failing to load properly (Code 31 errors).
I opened up the "Drivers" folder of the unpacked sp72926 and found a folder "I2C" there. Inside was a set of files including an .INF file. I right-clicked it, selected "Install" and voila! Sound and touch instantly returned, without even the need for a reboot (although I did one just to make sure).
No problems for the past several hours. Appears fixed to me!
Device Manager reports 5 listings of "I2C Controller", all by Intel, all version 604.10143.1010.2969, all dated 2015-06-08
When I updated the I2C driver using Device Manager, Windows Update downloaded and installed version 604.10154.1010.7150, dated 2015-10-01. Reboot will determine if it still works properly.
EDIT: The updated I2C driver appears to work well. Audio and touch fully operational. No other side effects. SOLVED!
EDIT: An explanation of HID over I2C (Inter-Integrated Circuit) can be found here:
https://msdn.microsoft.com/en-us/library/windows/hardware/jj127208(v=vs.85).aspx