To be specific, since upgrading to Windows 10 on both my machines (which were both previously running Audacity), only my microphones are recognized. Among the recording devices I see when right-clicking the speaker icon in the tray, I see only my webcame mic (default device), a Realtek Hi Def mic (not plugged in - there is no second mic), and Line In (not plugged in). There’s no “Stereo Mix” or anything else to represent my sound card.
What you do want to do is go to the web site of the computer or motherboard manufacturer and look for the latest Windows 10 audio drivers intended for your computer model.
When you have such Windows 10 drivers, or even if they are not available, I suggest you then right-click in empty space in Windows Sound where you see the Hi Def Mic and Line-In listed, and choose “Show Disabled Devices”. This should reveal Stereo Mix if your device drivers make it available.
Even without stereo mix you should be able to record computer playback in Audacity by changing to Windows WASAPI host in Device Toolbar then change to the (loopback) input for the playback device you are using.
I’m already using Windows 10, without any problem regarding sound or Audacity.
I tried Audacity 2.1.0 and it worked perfectly normal.
I just upgraded to v2.1.1 and everything seems fine too.
I think this might be useful for the developer to know.
As for the user asking for help, it certainly is somethinf to do with his/her own audio drivers.
Please do a Windows Update and try to check on Device Manager if all you audio devices are being properly recognised.
If those drivers are all ok, then on the notification area, find the sound icon and right click on it.
Then choose “Recording devices”, and set the default one accordingly. You may also change their settings here. This may be all you need to do.
Sometimes I change my video/audio output from my PC monitor to my TV, using HDMI.
And sometimes, after I change back to the PC, I need to change the default to the PC Card on the PC, because it got “stuck” on the TV. I don’t know if that is your case…