When tweaking Mate to look more like Gnome Classic, there is a simple way to move the volume control from the bottom panel to the top panel.
Bring up Mate’s menu panel by clicking on Menu in the lower-left corner of the screen. Search for and select Startup Applications, then scroll down and deselect Volume Control. When the machine is rebooted the volume control will no longer be shown in the lower-right corner of the screen. Now right-click on the top panel, select Add to Panel, then select the Volume Control. The volume control is now displayed in the top panel. Right-click on the icon, select Move and position the icon. Right-click on the icon once more and choose Lock to Panel.
That’s it. Effectively the volume control has been moved from the bottom panel to the top panel.
Two Audio controls when using Indicator-applet