Hi Brigitta,
Thank you for posting your query in Microsoft Community.
Download Conexant CX20751 SmartAudio HD Driver 8.66.4.0 for Windows 10 64 bit But it really depends on your type, the latest, but probably incompatible are here: Download Conexant Sound Card drivers. Download the latest drivers for your Conexant HD Audio to keep your Computer up-to-date. Supported OS: Windows 10 32 bit, Windows 8.1 32bit, Windows 7 32bit.
There are possibilities that not all the drivers may be compatible with Windows 10. However, you refer to the computer manufacturer's support website and check if there is any update on the sound driver and its compatibility with latest operating system (it could be Windows 8.1).
There could be a possibility of any third party software conflict as you have to restart the computer in order to hear the sound. I would suggest you to place the computer in clean boot state and check.
To help troubleshoot error messages and other issues, you can start Windows by using a minimal set of drivers and startup programs. This kind of startup is known as a 'clean boot.' A clean boot helps eliminate software conflicts.
Placing your system in Clean Boot state helps in identifying if any third party applications or startup items are causing the issue. You need to follow the steps from the article mentioned below to perform a Clean Boot.
How to perform a clean boot in Windows (information holds good for Windows 10 as well)
Note: Refer 'How to reset the computer to start normally after clean boot troubleshooting' to reset the computer to start as normal after troubleshooting.
Hope this information is helpful. Please feel free to reply in case you face any other issues with Windows in future.