I worked an issue in which several machines had the volume icon missing from the bottom right. I followed the steps to restore the sndvol32.exe from a working XP machine into a broken one and it worked.

Windows XP – Volume icon missing

I have XP Home,and for some reason,my volume control icon is missing from my taskbar. When I opened “Sounds and Audio Device Properties”,and checked the “place volume icon in the taskbar”,I get a pop-up announcing that “Windows cannot display the volume control on the taskbar because the volume control program has not been installed.To install,use add/remove programs in the control panel”
Where do I find this volume control program to install it?
This may all be my fault,as I have been trying to remove some freeware that stubbornly refuses to be unistalled,and also,I have been trying to modify the Startup menu so Thunderbird (which I recently installed to replace Outlook Express) would launch at Windows startup,but all that is another story.
Anyway,I sorely miss the volume control icon!

Before I sent this,I found what I thought was the solution in Windows Help and Support files:

Volume Icon Is Not Displayed in the Notification Area, and You Receive an Error Message When You Try to Add It
View products that this article applies to.
Article ID : 319095
Last Review : January 23, 2004
Revision : 1.0
This article was previously published under Q319095
SYMPTOMS
On a Windows XP-based computer, you may experience one or both of the following behaviors: • The Volume icon is no longer displayed in the notification area at the far right of the taskbar.
• When you try to add the volume control icon to the notification area (in the Sounds and Audio Devices utility in Control Panel, click to select the Place volume icon in the taskbar check box on the Volume tab), you receive the following error message:
Windows cannot display the volume control on the taskbar because the Volume Control program has not been installed. To install it use Add/Remove Programs in Control Panel.
As a result, you cannot start the Volume Control program and adjust the volume and sound level settings of your computer.
CAUSE
This behavior can occur if the Sndvol32.exe file is damaged or missing.
RESOLUTION
To resolve this issue, extract a new copy of Sndvol32.exe from the Windows XP compact disc to the system_root\System32 folder on your computer’s hard disk. To do this, follow these steps: 1. Insert the Windows XP CD-ROM into your computer’s CD-ROM or DVD-ROM drive. Press and hold down the SHIFT key as you insert the CD-ROM to prevent it from starting automatically.
2. Click Start, and then click Run.
3. In the Open box, type cmd, and then click OK.
4. Type the following lines at the command prompt, pressing ENTER after each line, where CD-ROM drive is the drive letter of the computer’s CD-ROM or DVD-ROM drive that contains the Windows XP CD-ROM, and system_root is the path and folder name where the Windows system files are located (generally C:\Windows):
CD-ROM drive:
cd i386
expand sndvol32.ex_ system_root\system32\sndvol32.exe

Note that there is a space between “ex_” and “system_root” in this command.

For example, if your computer’s CD-ROM drive is drive E, and the folder that contains the Windows system files is C:\Windows, type the following lines at the command prompt (pressing ENTER after each line):
e:
cd i386
expand sndvol32.ex_ c:\windows\system32\sndvol32.exe
5. Type exit to quit the command prompt.
6. Verify that you can start Volume Control and that the Volume icon is displayed in the notification area (if you selected this option).

MORE INFORMATION
By default, the Volume icon is not displayed in the notification area in Windows XP.

For additional information about how to display the volume control icon in the notification area, click the article number below to view the article in the Microsoft Knowledge Base:
279435 (http://support.microsoft.com/kb/279435/EN-US/) HOW TO: Display the Volume Icon in the Notification Area in Windows
For additional information about icons that are displayed in the notification area, click the article number below to view the article in the Microsoft Knowledge Base:
310578 (http://support.microsoft.com/kb/310578/EN-US/) Windows System Tray Icons
For additional information about how to troubleshoot sound problems, click the article number below to view the article in the Microsoft Knowledge Base:
307918 (http://support.microsoft.com/kb/307918/EN-US/) HOW TO: Resources for Troubleshooting Sound Problems in Windows XP