Are you running W95, W98 or Windows ME?
Usually if the speaker icon disappears from the System Tray, it's because Windows doesn't think you have a sound card installed or the drivers aren't installed or have become corrupted. You may just need to reinstall the drivers for the sound card but first do this ...
Click on Start>Settings>ControlPanel>Multimedia. What's selected in the box titled "Preferred Devices" in the "Playback" section? Also, click on that dropdown box and tell us what's available to be selected, if anything.
Also click on Start>Settings>CP>System>DeviceManager. Click on the "+" next to "Sound, Video and Games Controllers". Post the names of the devices you see listed underneath, if any.