There does not appear to be any way to set the left or right volume independently using the commands such as...
pacmd set-source-volume "source_name" 32000
It is currently only possible to set both left and right together.
What I am looking for is something like...
pacmd set-source-volume "source_name" 32000 40000
...where the two end parameters are left level and right level respectively.
FWIW, it's not just left+right, but any combination of channel maps.
pacmd is a debug tool which means it's not really important to support everything possible, but pactl should (IMO) support this somehow.
Changing "pacmd" to "pactl" in the bug title.
I've mailed the patch for this bug on the pulseaudio-discuss mailing list
see commit fc9fdfaf