...cannot play music files, and when I try to open the Volume Control, it says that "no mixer devices are installed." ...
I'm having the same problem. Nothing is red(X) or yellow(!) in my sound, video and game controllers. Here's a list of everything in Sound, Video, and Game controllers: Audio Codecs, Legacy Auido Drivers, Legacy Video Capture Devices, Media Control Devices, Realtek High Definition Audio, Video Codecs. All of these are enabled and working properly.
If I go to sounds and audio devices from the control panel: The volume tab says there is no audio device and all the settings are pretty much greyed out.
This just started happening today :(