With Win98, there will be a little 'speaker' icon in the lower right-hand corner (next to the clock). Clicking once on this will bring up a single volume control, double-clicking will bring up the main volume control window.
If your desktop has no such speaker icon, you might be able to access it through Start->Settings->Control Panel->Sound and multimedia. But if there is no speaker icon next to the clock, it's likely there is something wrong with the sound installation and it will have to be reinstalled.
If that's the case, take it to your nearest computer place and shell out the readies.