I decided to begin with something more basic.
I messed about with "alsamixer" and "arecord" and finally
arecord -d 10 -c 2 -t wav -f cd -D plug:dsnoop test.wav
Recording WAVE 'test.wav' : Signed 16 bit Little Endian, Rate 44100 Hz, Stereo
and "aplay test.wav" was successful
i.e. I can now record a wav from streaming web audio.
Now, back to mhWaveEdit..
Under "edit/preferences/sound/settings/recording device" it was defaulting to "hw".
I tried changing this to "plug:dsnoop". It allows this, but then when I try to record it says
"this format is not supported by the input driver" (whatever format I choose).