This topic refers not (!) to the already known bug.
I can't add the sound indicator to my xfce4 bar. There is no such item in the items list. And also - I have no icon for sound management in my system settings.
How to add the icon?
There isn't a Sound icon in Xubuntu 13.10 System Manager, instead just right-click the Audio Mixer applet in the panel and change the sound settings from there. To change the sound settings first install xfce4-mixer and then add the Audio Mixer applet to the panel.
Xfce4-mixer is the front-end for mixer settings delivered together with the Xfce4 desktop environment. It does the same jobs other mixer front-ends do, but is integrated into the Xfce4 desktop as a plugin for the Xfce4 main panel. To install xfce4-mixer from the terminal run:
Right-click on the panel and choose Panel -> Add New Items. From the Add New Items window, add the Audio Mixer applet, or drag the Audio Mixer applet icon from the Add New Items window to the panel.