pactl load-module module-null-sink channel_map=aux0,aux1,aux2,aux3
Thid doesn't fail:
pactl load-module module-null-sink channel_map=aux0,aux1,aux2,aux3 channels=4
Both should succeed. In the first case it's obvious that the intended amount of channels is 4. The problem is that pa_modargs_get_sample_spec_and_channel_map() fails if "channels" is not given and the default sample spec channels don't match the given channel map. pa_modargs_get_sample_spec_and_channel_map() needs to be smarter than that.
Patch sent to the mailing list: http://thread.gmane.org/gmane.comp.audio.pulseaudio.general/13205
Patch applied: http://cgit.freedesktop.org/pulseaudio/pulseaudio/commit/?id=8d0e9d4662d2fefc803921a607742588c12e8367