The sounds no longer is working.
Start with the obvious first, Control Panel, Sounds and audio devices, and check that the volume is up, for all items, and none have the "mute" box checked.
Next, go to the "sounds" tab, and where the various sounds are listed, highlight the first word, "asterisk" and click the triangle to the immediate right, it should play that sound annoyingly loud.
If not, and the "play" button does not release, thhen we may have a more serious problem.
Next, check the speakers are on, and plugged into the correct output (for powered speakers, the "line out" jack, for passive, the "headphones" jack.)
If your speakers are working, (this can be checked from the headphone jack of a CD, or mp3 player, for example), then the problem is the sound circuit.
Go to Control Panel, System, device manager, check "sounds and video controllers" make sure there is no yellow box with an exclamation mark next to "audio device." If there is one, then the drivers are not properly installed, or have become corrupted, go online and update to the latest drivers anyway.
If that yellow mark won't go away, or install fails for any reason, then the sound chip itself has died, or is sharing a resource with another device that doesn't like to be shared..
Also sometimes there is a headphone jack in the front of the PC, for easy access. If you plugged in a set, most often the rear jacks are disabled automatically. If the switch doesn't release when you unplug, the back speaker jacks will remain disabled.
If still under warranty, take it in to have the mainboard replaced, as the sound board is built in.
If not, then you can buy a USB sound device dongle, plug it into a USB slot, Plug your speakers into that, and you are good to go.
If you do pro audio, you shouldn't be using the onboard sound anyway, you can spend under a grand for a firewire interface that rivals any professional recording studio.
A word of caution, if the sound chip died, something must have killed it, like a spike or such. Other parts of the mainboard may not be far behind.
Think about replacing the mainboard, and use the opportunity to upgrade the machine.