Summarize this article:

By default, the Windows taskbar is at the bottom of the screen.  Right-clicking it displays a pop-up menu. It appears at the bottom of the pop-up menu that appears when you right-click of the taskbar. It's below the "Notification area." It's near the top of the menu.  This displays the volume icon in the taskbar notification area to the right.  If you still don't see the icon in the taskbar, click the icon that resembles a bracket pointing up on the left side of the notification area.  This displays all available icons in the taskbar. You can click and drag icons in the taskbar to re-arrange them.

Summary:
Right-click the Windows taskbar. Click Taskbar Settings. Scroll down and click Select which icons appear on the taskbar. Click the toggle switch  next to "Volume".