Sounds like you need to install the proper drivers for your sound card, for windows 98. You will need the soundcard cd disk for this to occur.
First check to make sure that your Sound Playback and Sound Recording devices are listed as the same device. Go to Control Panel, click on Multimedia and check the Audio (tab). If they are not listed the same, set them the same. If they are listed the same, use the drop down arrow next to where the device is named, and see if you have a second choice....
If you still cannot get sound, then read the below instructions.
If sound was once working for your windows 98 and windows 95 programs, but it stopped. It could be the driver became corrupted.
Go to your control panel, click on the System Icon, go to the Device Manager (tab). Scroll down to the topic of "Sound / Video & Game Controllers" . Open that topic and look for either ....
AC 97 Audio Controller
or
Legacy Audio Drivers
Highlight it and remove/uninstall it and restart windows. If the driver is still located on your hard drive, windows will relocate it and reload the driver, or you will be asked for your drivers disk.
Good Luck