You have lost your computer sound. Don't worry, the following will provide you some guides on getting your computer sound back.
(1) Your mistake in operating the BIOS may cause the problem: Due to different mother boards and versions of BIOS, there may have minor differences on changing the BIOS settings. Anyhow, the steps should be similar fundamentally like the following example: enter Award BIOS -> choose Integrated Peripherals -> select AC97 Audio and then change the Disable status into Auto status -> save the settings.
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(2) When you install the network card or other devices and then the sound card does not work anymore:
This case which is caused by compatibility issues or the interrupt conflict is very typical. It is easier to fix the conflict between drivers by updating all relative drivers. The interrupt conflict should be more complicate. You can fix the problem by performing these steps: enter Control Panel -> select system -> choose Hardware -> select Device Manager -> check all drivers' IRQ and then set IPQ manually to remove the interrupt conflict -> if you can not get your sound back after operating the above steps, you need to navigate to BIOS and then turn off some unnecessary devices so that you can spare some IRQ interrupt or you can change another slot for your network card or other devices.
(3) When you can play music via Windows Media Player or other Players installed on your computer but you can't listen to any video on the internet: It is very possible that it may cause by the incorrect settings and you can fix the problem by performing these steps: Open Internet Explorer -> click Tools -> select Internet Options -> click Advanced -> check whether you have enabled these two options "Play animations in web pages" and "play sounds in web pages".
My Computer Lost Sound - Guides on Getting Computer Sound Back
That is all. Hope you will fix your sound problem soon. You can also refer to Fix computer sound for more detailed solutions.