There's nothing more frustrating than loading up a video on a computer, only to see it start, but hear nothing. Sound failure on a computer can be caused by a variety of hardware and software issues, but many can be easily remedied if you can determine the cause.
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Check all the connections on your computer to make sure the speakers are plugged in, are receiving power, and that the sound card is properly seated in the motherboard.
Make sure the audio levels on your computer are turned up, your speaker volume is turned up, and the volume control on any specific programs you are using are turned up and not muted.
Bad or out-of-date software drivers can cause sound failure. Search online for the latest sound driver update for your sound card.
There's a chance that sound will fail due to blown out speakers, or speakers that are incompatible with a sound card. Test the speakers on a different computer or use different speakers to see if your speakers are causing an issue.
In some cases your sound card itself may be malfunctioning. Try uninstalling the sound card and testing it in a different computer, or using a different sound card.