Umm. Why install other apps?
Just put the standard xfce mixer volume control applet on the taskbar and:
1. If its the older version that has the volume represented by blue bars/arches on the icon, right click on it and specify the mixer to run: pavucontrol instead of xfce4-mixer, and select the controlled volume item (pulseaudio/mqster instead of alsa/master)
2. If its the newer one that on click shows a scroller, right click, select Playback:Built-in Audio... (or whatever you want if you have multiple sound cards), channel will be automatically master.
The below case may or may not apply since some disrtos seemingly changed the hardcoded mixer to the simplified pulse control:
Now comes the ugly part: since this applet has hardcoded xfce4-mixer as the "run mixer" shortcut, you may want to rename xfce4-mixer to, say xfce4-mixer-alsa, and link pavucontrol to xfce4-mixer:
sudo mv /usr/bin/xfce4-mixer /usr/bin/xfce4-mixer-alsa
sudo ln -s /usr/bin/pavucontrol /usr/bin/xfce4-mixer