Thanks for those.
H is the backup drive, a second SATA drive inside the case and on the motherboard. I moved the folder to the C drive. That made no difference.
Mixture's
start "ToggleAudio" /d "C
ToggleAudio\" /wait toggle-audio-7.exe
worked perfectly in "batch" mode, just like my batchfile.
It also locked up in exactly the same place as the batchfile when run by task scheduler.
I put in some echo > output.txt lines to track where the batchfile got to. It stopped at the line that starts the .exe file.
A separate shortcut to run the batch file did the same thing.
The .exe on its own in task scheduler was subtly different - the programme started and the audio control panel appeared on the screen, and stopped there. It didn't toggle over, although I could operate it as if I had opened it manually. The task scheduler showed "running" and task manager showed it running.
The problem seems to be that running the .exe from task scheduler (whether via a batch file or direct) it doesn't complete.
I have the batch file on a link on the desktop, and that works flawlessly every time.
Curious! I'm not that bothered about the action (the link on the desktop is enough), but I'm puzzled about the way it doesn't work.