Volume control applet disappeared

Solution 1:

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 in Ubuntu 12.04-14.04 run:

sudo apt-get install xfce4-mixer  

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.

There isn't a Sound icon in Xubuntu 13.10 Settings Manager, so if you want to change the sound settings, you have to right-click the Audio Mixer applet icon in the panel, and change the sound settings from there.