How to change the track controlled by xfce4-volumed

Xfce4-volumed currently doesn't have a graphical user interface to change the track it controls, however you can already do it by changing an xfconf key (by default, the daemon will grab the track reported as master by your card, or the first available track if there is no master track).

In a first time, you'll need to open xfce4-mixer and check the exact name of the track you want to control. I'll assume it is labelled "Master". This can change if your system is localized, however, or if you want to use a specific track.

Then, type the following command:
xfconf-query -c xfce4-mixer -p /active-track --create -t string -s "Master"

It'll set the "Master" track as the active track for xfce4-volumed. Note that if this track becomes unavailable in the future, then xfce4-volumed will just use the first available track again, until you manually reset the active track property to an existing track name.