Steve, many thanks for a speedy response.
In reply to your questions…
I wish to record directly from “PC Sounds”, e.g. via webpages - not particularly looking to record from CD/DVD nor via USB
Having said this, I have not got that far. As I mentioned before, the problem is in trying Setup in Preferences so, with this in mind the settings in “Device Toolbar” are:
Windows Direct Sound
… neither of these offer any devices
… this offers "Speakers (Realtek High Definition) Loop and “Realtek Digital Output”. On both of these, when I try to select Mono it gives the error message I reported in my previous message.
Stereo seems to work.
When using WASAPI, Audacity’s recording settings must match the device that you are recording from. This is because WASAPI gives a more direct connection between the device and the recording application, missing out format conversion.
The “Speakers (Realtek High Definition) Loopback” device is stereo, so Audacity must be set to record in stereo.
If you require a mono recording, record it as stereo, then convert the stereo track to mono.
There are two ways to convert a stereo track to mono.
Either you can “mix down” the two channels to a single mono track using “Tracks menu > Stereo Track to Mono” (use this if the stereo track has good audio in both channels), or, split the stereo track into two mono tracks, and delete the track that you don’t want. See here for how to split a stereo track to mono: http://manual.audacityteam.org/o/man/splitting_and_joining_stereo_tracks.html
I don’t receive that particular issue recording from WASAPI in mono on Windows 10 or 8.1, even though my playback devices only have stereo choices in “Default Format” in Windows Sound.
It could depend on your particular playback device, or it could be that making the playback device’s Default Format sample rate the same as the Audacity project rate, or enabling the “Exclusive Mode” boxes in Windows Sound will make mono recording less error prone.
Generally, WASAPI behaves better if Audacity project rate is 44100 Hz and both Exclusive Mode boxes for the playback device are on (checked). Exclusive Mode on “should” mean that the Default Format rate is ignored by Audacity and it just receives audio at the rate specified in the project rate, but that may not always be the case (another “bug”).
Restart Audacity after changing any settings in Windows Sound.