you need to add the audio-playback plug to get access to the pulse socket of the host and as sergio says, you can not run the pactl command from the host, just add the pulseaudio-utils stage package to have it available in your snap …
EDIT: oh, i see you are calling list sources, assuming you want to actually use the sources you find, that will also require the audio-record plug … note that this one needs to be manually connected after snap installation …