I configured a Lubuntu 16.04 system for a friend. While configuring we wanted to change sound settings. We clicked on the Lubuntu sound applet and on sound settings but nothing opened. I remeber that on some system I saw that pavucontrol opens this way. So we tried to install pavucontrol with pulseaudio but both was already installed. We started pavucontrol from terminal and it works. But terminal is absolutely no option for my friend. What is the problem with the applet and how to solve it?
EDIT: An answer describes a solution where the audio indicator is replaced by an audio applet. This solution works but I'm interested why the indicator doesn't work as expected and why I need this workaround.