In Startup Applications > Startup Programs list
click on first entry of Volume Control, click Edit
You should see this:
Click cancel, then click on the other instance of Volume Control and check if it is the same.
If they are ...
First (if you haven't already), remove the Volume Control you added in the panel (R-click, remove from panel).
Then, go back to Startup Appplications.
Click one Volume Control entry from the list, and click Remove, close ... reboot.
If it doesn't appear in the Notification area applet, remove the remaining Volume Control entry in the list in the same way, reboot.
Then go back to Startup Applications > Startup Program list ... click Add, and recreate the entry by copying the exact information as shown in the Edit box picture above.
The command line in full is mate-volume-control-applet
Click Save, close, reboot.
Desktop: C2D E6400 / Asus P5LD2SE / 2GB ram / Asus GeForce 210 / 320GB (WinXP) & 1TB (Mint - various)
Laptop: Asus A53E-SX1455V - i5 2450M / 4GB ram / Intel HD 3000 / 500GB (Win7/Mint 17.3 Cinnamon)