Hi Jan. I'm Greg, a volunteer installation specialist and 8 year Windows MVP, here to help you.
Please run the Sound Playback troubleshooter at Settings>Update & Security>Troubleshoot.
Also right click the Volume icon in System Tray, choose Recording Devices. Is microphone detected? Try changing USB ports until it registers there, click on it to Set as Default, Configure if necessary.
Update the Sound driver from the PC's Support Downloads web page.
If no newer driver then uninstall the one presently installed in Device Manager, reached by right clicking the Start Button. Restart PC to reinstall.
It's possible this was caused by a Windows Update so if nothing above works, go to Settings>Update & Security>Windows Update, check the Installed Updates list for Updates that might have been just installed, try uninstalling them using the Uninstall button present there. Then hide the Updates using the Hide Updates tool downloaded from here: http://download.microsoft.com/download/f/2/2/f2...
If no Updates listed this could also be the latest Windows 10 Version 1709 Update which you can check at Settings>System>About and if necessary roll back at Settings>Update & Security>Recovery, then hide with the tool given above to wait for it to mature.
Or you can try the most stable method to update to the latest version (which you need anyway) by installing Media Creation Tool and then choosing to Upgrade Now: http://windows.microsoft.com/en-us/windows-10/m...
I hope this helps. Feel free to ask back any questions and let us know how it goes. I will keep working with you until it's resolved.