Been doing some digging, looked like I can set a default in /etc/pulse/client.conf but it doesn't do anything on reboot. There's also /lib/udev/rules.d/90-pulseaudio but that has a warning that it will be overwritten on update so that's not the place. /usr/share/pulseaudio/alsa-mixer/profile-sets contains a default.conf but that contains different types of sound rather than linking it to a specific app (unless a call contains a specific type of sound?) Noting these so I know where not to look tomorrow as much as anything as I'm not expecting anyone to have played with this.