I use the Windows 10 system, so i do not know the details but i know how to get it back on Windows 10. I think it will applies to Windows 7.
Right-click your taskbar and choose taskbar settings > Notification area > Select which icon appear on the taskbar. Then choose the Volume statu as On.
And another way you can fix it is restart the Windows Explorer. Right-click the taskbar and choose Task Manager. In Process tab, find the Windows Explorer to restart it.