Xfce4-Volumed is responsible of making the volume up/down and mute keys of the keyboard work automatically, and uses the Xfce 4 audio mixer’s defined card and track for chosing which track to act on. It also provides volume change / mute toggle notifications if the notification server used supports x-canonical-icon-only and x-canonical-synchronous notifications.
This daemon is (a very tiny) part of Xfce 4. Pulseaudio users may want to fork the https://launchpad.net/ubuntu/+source/xfce4-volumed. Thanks to Jannis, Jérôme, Lionel and others for their interest and insights.
Xfce4-Volumed (and possibly the audio mixer maintained by Jannis) is in high need of a new maintainer, as I don’t have the time to look into GStreamer-related bugs. Get in touch with the Xfce folks if you want to contribute to Free and Open Source Software!