For some unknown reason, the audio volume control icon and network control icon disappeared from the MATE's desktop panel. I am using 64-bit CentOS 7.X. I googled extensively the net. Many people complained but, unfortunately, no real solution. My system is up-to-date-ed. The missing icons do not exist in the MATE Configuration Tool. I found there the "sound" icon. In order to change the audio volume, I have to click to the icon and then slide the volume bar. What I would like to do is to use mouse wheel while the mouse pointer hover on top of the volume icon.

In other words, I would like to get the original 'volume control' and 'network control' icon back to the MATE's desktop top panel. The question is "how?".